(split out from #30207)
https://doc.sagemath.org/html/en/installation/source.html#installation-in-a-multiuser-environment should be updated:
- Don't edit the script
- Just symlink to the sagescript in venv
We also add a link from the README to an optional step described only in the installation manual.
CC:  @dimpase @jhpalmieri @slel
Component: documentation
Author: Matthias Koeppe
Branch/Commit: 7bc3394
Reviewer: John Palmieri
Issue created by migration from https://trac.sagemath.org/ticket/33787