Add document explaining the authentication system

## Requests for new Topics

> Write a **short** paragraph describing what the topic will cover.

Today our authentication documentation is scenario focused. This is fine but doesn't help explain the fundamentals of the system and the various component responsibilities. We should have an introduction to the authentication system that describes the services (IAuthentictation* interfaces), the verbs (challenge, authentication, forbid, signin, signout), what authentication schemes are, what authentication handlers are etc. There should also be diagrams of a typical authentication flow with all of the actors involved.

> 2. Write an outline of the proposed topic contents.

Here's an example of what I think this topic could look like:
https://digitalmccullough.com/posts/aspnetcore-auth-system-demystified.html

> 3. Where would you put the topic in the Table of Contents.

https://docs.microsoft.com/en-us/aspnet/core/security/authentication/

> 4. If you know who should review the issue, notify them by including @githubID in the issue description.

@HaoK @blowdart @Tratcher