Document authentication

[askvortsov1]

One of the more common support requests is from people looking to integrate their flarum forum with the rest of their site through shared user accounts / authentication. A documentation article explaining:

• How Flarum authentication currently works
• Flarum's APIs for federated / social login
• How Flarum might be extended to offboard authentication to an external server (or moreso, how global login might be implemented)

would go a huge way