Various improvements to quick-install document
Various improvements to quick-install document

- removed paragraph about old Debian Wheezy, the link is broken, and Wheezy doesn't even get security updates anymore, so seems unlikely anyone would do a new install with it.
- clarified that Let's Encrypt script uses only the HTTP challenge.
- added links to a few things that newbies might want to look up (nginx, apache, jetty, SIP, FQDN, Let's Encrypt, etc.
- added some basic debugging starting points, based on my experience
- some minor grammatical tweaks
- other minor tweaks

gpolitis/jitsi-meet,jitsi/jitsi-meet,jitsi/jitsi-meet,bgrozev/jitsi-meet,gpolitis/jitsi-meet,gpolitis/jitsi-meet,bgrozev/jitsi-meet,jitsi/jitsi-meet,gpolitis/jitsi-meet,jitsi/jitsi-meet,bgrozev/jitsi-meet,jitsi/jitsi-meet,gpolitis/jitsi-meet,jitsi/jitsi-meet,jitsi/jitsi-meet,bgrozev/jitsi-meet,bgrozev/jitsi-meet,gpolitis/jitsi-meet,gpolitis/jitsi-meet,bgrozev/jitsi-meet,gpolitis/jitsi-meet,jitsi/jitsi-meet