doc: Update for FreeBSD 12.2, add GUI Build Instructions
doc: Update for FreeBSD 12.2, add GUI Build Instructions

The current FreeBSD Build documentation is a little outdated and underwhelming. This PR updates the doc to
be more informative. It also adds new instructions for building the GUI and adding support for  descriptor
wallets.

on vasild's recommendation: it is ok to point the user to download db5 which has an active port package
instead of building db4 from the provided script.

Co-authored-by: Vasil Dimov <ad45205a471752ff1e17b788c3e52521e319899e@freebsd.org>

jamesob/bitcoin,pataquets/namecoin-core,namecoin/namecoin-core,ajtowns/bitcoin,domob1812/bitcoin,bitcoinsSG/bitcoin,jambolo/bitcoin,Sjors/bitcoin,anditto/bitcoin,ajtowns/bitcoin,Xekyo/bitcoin,practicalswift/bitcoin,Sjors/bitcoin,dscotese/bitcoin,n1bor/bitcoin,domob1812/namecore,practicalswift/bitcoin,sipsorcery/bitcoin,MarcoFalke/bitcoin,mruddy/bitcoin,practicalswift/bitcoin,ElementsProject/elements,bitcoinknots/bitcoin,ajtowns/bitcoin,andreaskern/bitcoin,MeshCollider/bitcoin,cdecker/bitcoin,yenliangl/bitcoin,jambolo/bitcoin,n1bor/bitcoin,rnicoll/bitcoin,mruddy/bitcoin,dscotese/bitcoin,jambolo/bitcoin,namecoin/namecore,qtumproject/qtum,AkioNak/bitcoin,lateminer/bitcoin,MeshCollider/bitcoin,lateminer/bitcoin,sipsorcery/bitcoin,ElementsProject/elements,domob1812/bitcoin,instagibbs/bitcoin,ElementsProject/elements,jambolo/bitcoin,jnewbery/bitcoin,MarcoFalke/bitcoin,sipsorcery/bitcoin,yenliangl/bitcoin,yenliangl/bitcoin,sipsorcery/bitcoin,cdecker/bitcoin,GroestlCoin/GroestlCoin,prusnak/bitcoin,bitcoin/bitcoin,fanquake/bitcoin,n1bor/bitcoin,jamesob/bitcoin,prusnak/bitcoin,namecoin/namecore,MarcoFalke/bitcoin,n1bor/bitcoin,pstratem/bitcoin,mruddy/bitcoin,mm-s/bitcoin,jamesob/bitcoin,andreaskern/bitcoin,mruddy/bitcoin,pataquets/namecoin-core,namecoin/namecoin-core,pstratem/bitcoin,fujicoin/fujicoin,n1bor/bitcoin,prusnak/bitcoin,mm-s/bitcoin,GroestlCoin/GroestlCoin,apoelstra/bitcoin,domob1812/namecore,ElementsProject/elements,bitcoin/bitcoin,EthanHeilman/bitcoin,mm-s/bitcoin,GroestlCoin/GroestlCoin,rnicoll/bitcoin,particl/particl-core,ElementsProject/elements,pataquets/namecoin-core,n1bor/bitcoin,practicalswift/bitcoin,sstone/bitcoin,domob1812/namecore,GroestlCoin/bitcoin,fujicoin/fujicoin,jlopp/statoshi,MarcoFalke/bitcoin,bitcoinsSG/bitcoin,pstratem/bitcoin,namecoin/namecoin-core,andreaskern/bitcoin,yenliangl/bitcoin,bitcoin/bitcoin,Sjors/bitcoin,lateminer/bitcoin,fujicoin/fujicoin,qtumproject/qtum,ajtowns/bitcoin,tecnovert/particl-core,dscotese/bitcoin,fujicoin/fujicoin,tecnovert/particl-core,particl/particl-core,prusnak/bitcoin,EthanHeilman/bitcoin,sstone/bitcoin,kallewoof/bitcoin,apoelstra/bitcoin,kallewoof/bitcoin,domob1812/bitcoin,mm-s/bitcoin,mruddy/bitcoin,MeshCollider/bitcoin,bitcoinsSG/bitcoin,EthanHeilman/bitcoin,GroestlCoin/bitcoin,jambolo/bitcoin,jnewbery/bitcoin,practicalswift/bitcoin,namecoin/namecoin-core,qtumproject/qtum,sstone/bitcoin,pataquets/namecoin-core,GroestlCoin/bitcoin,GroestlCoin/GroestlCoin,anditto/bitcoin,GroestlCoin/GroestlCoin,bitcoin/bitcoin,tecnovert/particl-core,prusnak/bitcoin,JeremyRubin/bitcoin,yenliangl/bitcoin,mm-s/bitcoin,yenliangl/bitcoin,rnicoll/bitcoin,jnewbery/bitcoin,AkioNak/bitcoin,kallewoof/bitcoin,MarcoFalke/bitcoin,rnicoll/bitcoin,practicalswift/bitcoin,qtumproject/qtum,fanquake/bitcoin,kallewoof/bitcoin,instagibbs/bitcoin,MarcoFalke/bitcoin,tecnovert/particl-core,lateminer/bitcoin,EthanHeilman/bitcoin,namecoin/namecore,jamesob/bitcoin,pataquets/namecoin-core,fanquake/bitcoin,domob1812/bitcoin,AkioNak/bitcoin,instagibbs/bitcoin,Xekyo/bitcoin,dscotese/bitcoin,Xekyo/bitcoin,Xekyo/bitcoin,JeremyRubin/bitcoin,rnicoll/bitcoin,apoelstra/bitcoin,ElementsProject/elements,domob1812/namecore,Sjors/bitcoin,achow101/bitcoin,pstratem/bitcoin,sstone/bitcoin,lateminer/bitcoin,pstratem/bitcoin,tecnovert/particl-core,fanquake/bitcoin,tecnovert/particl-core,apoelstra/bitcoin,GroestlCoin/bitcoin,anditto/bitcoin,AkioNak/bitcoin,anditto/bitcoin,bitcoinknots/bitcoin,AkioNak/bitcoin,fujicoin/fujicoin,bitcoinsSG/bitcoin,bitcoinsSG/bitcoin,namecoin/namecore,instagibbs/bitcoin,jamesob/bitcoin,jlopp/statoshi,rnicoll/bitcoin,qtumproject/qtum,jamesob/bitcoin,bitcoin/bitcoin,Sjors/bitcoin,Xekyo/bitcoin,jlopp/statoshi,MeshCollider/bitcoin,achow101/bitcoin,particl/particl-core,JeremyRubin/bitcoin,bitcoinknots/bitcoin,ajtowns/bitcoin,domob1812/namecore,sstone/bitcoin,anditto/bitcoin,qtumproject/qtum,JeremyRubin/bitcoin,andreaskern/bitcoin,cdecker/bitcoin,mm-s/bitcoin,apoelstra/bitcoin,achow101/bitcoin,sipsorcery/bitcoin,jlopp/statoshi,GroestlCoin/GroestlCoin,jlopp/statoshi,sstone/bitcoin,GroestlCoin/bitcoin,ajtowns/bitcoin,jambolo/bitcoin,particl/particl-core,bitcoinsSG/bitcoin,Xekyo/bitcoin,domob1812/bitcoin,namecoin/namecore,apoelstra/bitcoin,instagibbs/bitcoin,namecoin/namecoin-core,EthanHeilman/bitcoin,JeremyRubin/bitcoin,prusnak/bitcoin,fanquake/bitcoin,achow101/bitcoin,achow101/bitcoin,sipsorcery/bitcoin,pataquets/namecoin-core,MeshCollider/bitcoin,AkioNak/bitcoin,cdecker/bitcoin,jnewbery/bitcoin,particl/particl-core,instagibbs/bitcoin,dscotese/bitcoin,domob1812/bitcoin,GroestlCoin/bitcoin,bitcoinknots/bitcoin,dscotese/bitcoin,cdecker/bitcoin,bitcoin/bitcoin,JeremyRubin/bitcoin,EthanHeilman/bitcoin,namecoin/namecoin-core,MeshCollider/bitcoin,mruddy/bitcoin,achow101/bitcoin,fanquake/bitcoin,bitcoinknots/bitcoin,pstratem/bitcoin,particl/particl-core,lateminer/bitcoin,andreaskern/bitcoin,qtumproject/qtum,domob1812/namecore,jnewbery/bitcoin,jlopp/statoshi,anditto/bitcoin,andreaskern/bitcoin,namec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