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

[beanzmo]

@nirvn I would suggest to have a video here. This is not so easy. And perhaps, we could give one or two examples of where this would be useful to implement?

[nirvn]

@beanzmo , could we merge this as is, and eventually add a video? It'll be hard to make one to begin with, we need a SSO WFS/WMS server, etc.

[nirvn]

@m-kuhn , thank(s) 😉 I went way overboard with these (s).