minor #20203 3.0 Upgrade Guide: Added Table of Contents (smithandre)
minor #20203 3.0 Upgrade Guide: Added Table of Contents (smithandre)

This PR was submitted for the master branch but it was merged into the 3.1 branch instead (closes #20203).

Discussion
----------

3.0 Upgrade Guide: Added Table of Contents

| Q             | A
| ------------- | ---
| Branch?       | master
| Bug fix?      | no
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets |
| License       | MIT
| Doc PR        | NA

Added a table of contents, which helps me find relevant upgrade information a bit faster.  It's very basic and straight forward, as I don't want to add too much overhead in terms of maintaining these guides.

Commits
-------

3a458d3 3.0 Upgrade Guide: Added Table of Contents

mnapoli/symfony,WhiteEagle88/symfony,sstok/symfony,michaelperrin/symfony,jean-gui/symfony,vincentaubert/symfony,garak/symfony,hacfi/symfony,Padam87/symfony,garak/symfony,Triiistan/symfony,Triiistan/symfony,voronkovich/symfony,jvasseur/symfony,chalasr/symfony,smatyas/symfony,pyrech/symfony,klandaika/symfony,Simperfit/symfony,Slamdunk/symfony,Pierstoval/symfony,1ed/symfony,jean-gui/symfony,zorn-v/symfony,Slamdunk/symfony,Ener-Getick/symfony,Slamdunk/symfony,Einenlum/symfony,Deamon/symfony,tgalopin/symfony,gencer/symfony,romaricdrigon/symfony,Pierstoval/symfony,michaelperrin/symfony,apfelbox/symfony,apetitpa/symfony,mpdude/symfony,lyrixx/symfony,joelwurtz/symfony,pyrech/symfony,mweimerskirch/symfony,localheinz/symfony,mweimerskirch/symfony,sroze/symfony,arjenm/symfony,csarrazi/symfony,joelwurtz/symfony,DavidGarciaCat/symfony,pierredup/symfony,codeclimate-testing/symfony,docteurklein/symfony,curry684/symfony,alekitto/symfony,lstrojny/symfony,Engerim/symfony,Amo/symfony,joelwurtz/symfony,docteurklein/symfony,gimler/symfony,jjanvier/symfony,dosten/symfony,weaverryan/symfony,jameshalsall/symfony,andreia/symfony,jjanvier/symfony,apfelbox/symfony,ro0NL/symfony,teohhanhui/symfony,zcodes/symfony,k0pernikus/symfony,ajgarlag/symfony,teohhanhui/symfony,jjanvier/symfony,Aliance/symfony,neemzy/symfony,voronkovich/symfony,frankdejonge/symfony,lyrixx/symfony,Einenlum/symfony,Padam87/symfony,dunglas/symfony,nicolas-grekas/symfony,yceruto/symfony,MatTheCat/symfony,backbone87/symfony,nasimnabavi/symfony,vincentaubert/symfony,stof/symfony,Tobion/symfony,andrerom/symfony,ajgarlag/symfony,apfelbox/symfony,e-moe/symfony,dmaicher/symfony,Koc/symfony,pierredup/symfony,maidmaid/symfony,pierredup/symfony,Taluu/symfony,ro0NL/symfony,maidmaid/symfony,nicolas-grekas/symfony,shieldo/symfony,Einenlum/symfony,hhamon/symfony,d-ph/symfony,GuilhemN/symfony,jzawadzki/symfony,Pierstoval/symfony,andreia/symfony,Deamon/symfony,maidmaid/symfony,michaelperrin/symfony,tucksaun/symfony,sstok/symfony,DavidGarciaCat/symfony,dunglas/symfony,garak/symfony,codeclimate-testing/symfony,mweimerskirch/symfony,symfony/symfony,andreia/symfony,Strate/symfony,weaverryan/symfony,paradajozsef/symfony,c960657/symfony,hhamon/symfony,curry684/symfony,DavidGarciaCat/symfony,dmaicher/symfony,damienalexandre/symfony,romaricdrigon/symfony,tucksaun/symfony,ragboyjr/symfony,Tobion/symfony,dbu/symfony,phramz/symfony,Tobion/symfony,alekitto/symfony,mweimerskirch/symfony,Aliance/symfony,kbond/symfony,bendavies/symfony,sstok/symfony,romaricdrigon/symfony,apfelbox/symfony,localheinz/symfony,Ener-Getick/symfony,nicolas-grekas/symfony,Simperfit/symfony,zcodes/symfony,deguif/symfony,shieldo/symfony,Nyholm/symfony,zerkms/symfony,chrisguitarguy/symfony,mnapoli/symfony,sgehrig/symfony,dzuelke/symfony,mweimerskirch/symfony,yceruto/symfony,kbond/symfony,MatTheCat/symfony,zerkms/symfony,GuilhemN/symfony,e-moe/symfony,hason/symfony,okdana/symfony,sroze/symfony,romainneutron/symfony,dmaicher/symfony,chrisguitarguy/symfony,mnapoli/symfony,arjenm/symfony,Triiistan/symfony,Padam87/symfony,docteurklein/symfony,Strate/symfony,Deamon/symfony,andrerom/symfony,frankdejonge/symfony,c960657/symfony,gencer/symfony,symfony/symfony,Slamdunk/symfony,mpdude/symfony,HeahDude/symfony,josephdpurcell/symfony,ragboyjr/symfony,k0pernikus/symfony,Tobion/symfony,jderusse/symfony,backbone87/symfony,dbu/symfony,iltar/symfony,dbu/symfony,jameshalsall/symfony,neemzy/symfony,damienalexandre/symfony,chrisguitarguy/symfony,apetitpa/symfony,jderusse/symfony,neemzy/symfony,jzawadzki/symfony,stof/symfony,sroze/symfony,e-moe/symfony,ajgarlag/symfony,josephdpurcell/symfony,patrick-mcdougle/symfony,sgehrig/symfony,Amo/symfony,Koc/symfony,lyrixx/symfony,stof/symfony,tgalopin/symfony,Koc/symfony,jakzal/symfony,patrick-mcdougle/symfony,ragboyjr/symfony,nasimnabavi/symfony,lepiaf/symfony,pyrech/symfony,e-moe/symfony,romainneutron/symfony,Strate/symfony,phramz/symfony,tucksaun/symfony,xabbuh/symfony,teohhanhui/symfony,d-ph/symfony,tucksaun/symfony,sroze/symfony,gimler/symfony,symfony/symfony,smatyas/symfony,dosten/symfony,chalasr/symfony,jakzal/symfony,paradajozsef/symfony,sgehrig/symfony,docteurklein/symfony,gencer/symfony,jderusse/symfony,k0pernikus/symfony,vincentaubert/symfony,lepiaf/symfony,chrisguitarguy/symfony,jakzal/symfony,OskarStark/symfony,ro0NL/symfony,gnat42/symfony,klandaika/symfony,nasimnabavi/symfony,apfelbox/symfony,zerkms/symfony,fabpot/symfony,symfony/symfony,chalasr/symfony,gonzalovilaseca/symfony,bendavies/symfony,zorn-v/symfony,jjanvier/symfony,curry684/symfony,Ener-Getick/symfony,hason/symfony,csarrazi/symfony,Simperfit/symfony,jzawadzki/symfony,GuilhemN/symfony,romainneutron/symfony,shieldo/symfony,localheinz/symfony,damienalexandre/symfony,jameshalsall/symfony,phramz/symfony,oleg-andreyev/symfony,mpdude/symfony,maidmaid/symfony,k0pernikus/symfony,pamil/symfony,pamil/symfony,Simperfit/symfony,kbond/symfony,jakzal/symfony,jean-gui/symfony,andrerom/symfony,phramz/symfony,aeoris/symfony,dunglas/symfony,nasimnabavi/symfony,jameshalsall/symfony,jean-gui/symfony,lyrixx/symfony,pierredup/symfony,docteurklein/symfony,arjenm/symfony,yceruto/symfony,frankdejonge/symfony,patrick-mcdougle/symfony,stof/symfony,MatTheCat/symfony,ogizanagi/symfony,jvasseur/symfony,weaverryan/symfony,jderusse/symfony,tgalopin/symfony,Engerim/symfony,dzuelke/symfony,pamil/symfony,oleg-andreyev/symfony,Padam87/symfony,zorn-v/symfony,okdana/symfony,Ener-Getick/symfony,pamil/symfony,klandaika/symfony,Nyholm/symfony,ogizanagi/symfony,deguif/symfony,1ed/symfony,gonzalovilaseca/symfony,derrabus/symfony,ogizanagi/symfony,chrisguitarguy/symfony,c960657/symfony,teohhanhui/symfony,mpdude/symfony,localheinz/symfony,d-ph/symfony,Nyholm/symfony,derrabus/symfony,apetitpa/symfony,xabbuh/symfony,WhiteEagle88/symfony,Ener-Getick/symfony,andreia/symfony,Ener-Getick/symfony,aeoris/symfony,Simperfit/symfony,OskarStark/symfony,frankdejonge/symfony,GuilhemN/symfony,zerkms/symfony,Pierstoval/symfony,joelwurtz/symfony,sstok/symfony,arjenm/symfony,Taluu/symfony,deguif/symfony,dosten/symfony,hhamon/symfony,fabpot/symfony,Deamon/symfony,michaelperrin/symfony,hacfi/symfony,alekitto/symfony,jjanvier/symfony,Engerim/symfony,shieldo/symfony,gimler/symfony,gnat42/symfony,kbond/symfony,lstrojny/symfony,damienalexandre/symfony,klandaika/symfony,dosten/symfony,romaricdrigon/symfony,yceruto/symfony,dbu/symfony,Amo/symfony,backbone87/symfony,vincentaubert/symfony,HeahDude/symfony,HeahDude/symfony,nicolas-grekas/symfony,gnat42/symfony,fabpot/symfony,smatyas/symfony,bendavies/symfony,fabpot/symfony,WhiteEagle88/symfony,csarrazi/symfony,paradajozsef/symfony,ajgarlag/symfony,dzuelke/symfony,OskarStark/symfony,c960657/symfony,mnapoli/symfony,gonzalovilaseca/symfony,zcodes/symfony,gnat42/symfony,jameshalsall/symfony,Triiistan/symfony,iltar/symfony,Aliance/symfony,klandaika/symfony,k0pernikus/symfony,GuilhemN/symfony,Deamon/symfony,voronkovich/symfony,1ed/symfony,Padam87/symfony,hhamon/symfony,garak/symfony,Einenlum/symfony,josephdpurcell/symfony,ogizanagi/symfony,ragboyjr/symfony,jvasseur/symfony,oleg-andreyev/symfony,HeahDude/symfony,smatyas/symfony,mnapoli/symfony,curry684/symfony,sgehrig/symfony,apetitpa/symfony,dmaicher/symfony,OskarStark/symfony,weaverryan/symfony,dzuelke/symfony,bendavies/symfony,zcodes/symfony,deguif/symfony,gonzalovilaseca/symfony,sstok/symfony,patrick-mcdougle/symfony,xabbuh/symfony,chalasr/symfony,voronkovich/symfony,romainneutron/symfony,lepiaf/symfony,ro0NL/symfony,aeoris/symfony,MatTheCat/symfony,hason/symfony,romainneutron/symfony,d-ph/symfony,oleg-andreyev/symfony,Taluu/symfony,dbu/symfony,Taluu/symfony,jvasseur/symfony,codeclimate-testing/symfony,iltar/symfony,derrabus/symfony,Nyholm/symfony,tgalopin/symfony,okdana/symfony,Koc/symfony,Triiistan/symfony,andrerom/symfony,smatyas/symfony,paradajozsef/symfony,Amo/symfony,xabbuh/symfony,derrabus/symfony,gimler/symfony,zorn-v/symfony,Amo/symfony,dzuelke/symfony,alekitto/symfony,lstrojny/symfony,dosten/symfony,pyrech/symfony,codeclimate-testing/symfony,iltar/symfony,1ed/symfony,hacfi/symfony,patrick-mcdougle/symfony