Auth docs are missing SignIn/SignOut part.

**Describe the new topic**

I had to look inside the source code to understand how `SignIn` and `SignOut` flows work, and which party needs to implement the logic. (Docs do not mention `IAuthenticationSignInHandler`). Describing what `SignIn` and `SignOut` are intended for would be useful.

Also, I could not find a place where docs talk about `IAuthenticationRequestHandler` and what role it plays in authentication. Especially for `RemoteAuthenticationHandler` implementations which expect callback from remote identity providers.

IMO, this is an important subject related to authentication which should be added to the documentation.