Documentation: Update Readme to reflect admin page + document usage with a few providers (keycloak, authelia) about jellyfin-plugin-sso

[strazto]

quick authentik update:

You can add the group membership scope for role claim by adding a custom expression based property mapping:

return [group.name for group in user.ak_groups.all()]

[9p4]

It's a similar flow for many OpenID programs, so people should be able to figure it out. Maybe another file with provider-specific documentation would be best. I would prefer not to use the wiki because interoperability/lock in

[9p4]

I can help out with Google and some other providers I've tested. I also have access to Azure so I can add some documentation regarding that as well.