Document the authenticators available from the jupyterhub package

[GeorgianaElena]

Originally reported by @ReubenTheDS in https://github.com/jupyterhub/the-littlest-jupyterhub/issues/638.

Proposed change

Add some info in the docs about the authenticators that ship with jupyterhub by default and thus can be used by TLJH without installing anything else:

• LocalAuthenticator
• PAMAuthenticator
• DummyAuthenticator

Also, the DummyAuthenticator used to be part of its own package, but has since been added to the jupyterhub package itself. We should update the docs with this info and stop installing the jupyterhub-dummyauthenticator package.

[consideRatio]

Also, the DummyAuthenticator used to be part of its own package, but has since been added to the jupyterhub package itself. We should update the docs with this info and stop installing the jupyterhub-dummyauthenticator package.

I've addressed this part of the issue in #721 and #719.