Improve the authentication documentation

[nirvn]

Improvements include:

• moving away from being an OAuth2-specific page; we support all authentication methods and the current page makes it hard to understand we do
• removing a couple of "she" usages in favor of "you".
• removing mention of QGIS 3.20, as minimum version, we're many years into having moved away from that version, we can lighten that part of the documentation