Update User Authentication docs with steps on how to restrict user sign-up and sign-in

Created by: miveronese

#35676

This PR updates the User Authentication section, under Administration, with information on how to use the new (and also old) site configuration fields to control sign in and sign up to a Sourcegraph instance through different auth providers.

Test plan

• Accessed the documentation area via Site Config > Documentation in a local instance.
• Tested the links and searched the new content