Let's improve the installation-from-source documentation regarding the "recommended system packages" for 9.1.

• build/pkgs/_recommended - records additional recommended packages such as texlive-xetex (see https://wiki.sagemath.org/prerequisites/Ubuntu); also texinfo (from update ECL to 20.4.24 #22191)

(from #29124)

Related:

• Meta-ticket: Refactor and improve system package related scripts, tox.ini, build/bin/write_dockerfile.sh #29146: Meta-ticket: Refactor and improve system package related scripts, tox.ini, build/bin/write_dockerfile.sh
• At the end of configure, show installation hints for non-enabled optional packages separately #29372: At the end of configure, show installation hints for disabled optional packages separately
• Improve wording and formatting of configure's recommendation message #30624: Improve configure's recommendation message

CC: @egourgoulhon @dimpase @kiwifb @orlitzky @jhpalmieri @videlec @kcrisman @slel @antonio-rojas @kliem @thierry-FreeBSD @saraedum

Component: documentation

Keywords: sd109

Author: Matthias Koeppe, John Palmieri

Branch: 6d5cc43

Reviewer: John Palmieri, Matthias Koeppe

Issue created by migration from https://trac.sagemath.org/ticket/29557