This updates our documentation to rework the guidance for installing

Review Board to prioritize the new Review Board Installer.
The new page for this covers the system requirements, running the

installer, advanced options for unattended installs and proxies, and

then guiding to the appropriate place for completing a site

installation.
A page listing all compatible Linux versions is available, and linked to

on the installer and old Linux pages.
The Linux, Windows, and macOS pages have been updated to reference the

new installer docs as well.
The database versions in the new and existing compatibility lists have

also been bumped up to modern versions that we'll want to push going

forward, based on Django's deprecation cycle.
This should hopefully do a lot to help new users get started with

Review Board.

Built the docs. Checked for build errors, spelling errors, and bad

links.
Read through it several times, looking for issues or confusion in

the new docs. It needs another pair of eyes.