Document OIDC OAuth

[brandonkal]

Description

Gitea should document this. It is noticeably missing here. It is not in the app.ini file either.

try.gitea.com has a login with GitHub button in the UI, so it should be possible.

Edit: I see it is possible via settings. This should be mentioned on the corresponding doc pages.

[stale]

This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs during the next 2 weeks. Thank you for your contributions.

[Morriz]

bump...where can we find docs on how to use OIDC properly? Such as how to configure the offered groups claim to map to incoming teams?