Document which PAM apps we explicitly support

[adombeck]

I noticed in https://github.com/ubuntu/authd/pull/599#discussion_r1831242321 that we might want to consider the impact of changes to the PAM module on more apps than just SSH and GDM.

Lets document somewhere which PAM apps we explicitly support, and which other PAM apps might work as well (without promising support), to make both our lives easier when designing and testing changes like the PR linked above and to make it clear to users what they can expect.

Maybe https://github.com/ubuntu/authd/wiki/01---Get-started-with-authd is a good place for that.