Add Temporary Authenticator documentation

[ktaletsk]

This adds another How-To chapter on how to set up tmpauthenticator.

In my experience wanting to set this up with existing docs, the configuration was not immediately obvious.
For example, in JupyterHub Authenticator.allow_all is false by default and I was getting error Failed login for unknown user. However, coming from working with JupyterHub config files, it was not clear how to set this up in TLJH. Turns out, you need

tljh-config set auth.TmpAuthenticator.allow_all 'True'

I hope this additional documentation is helpful and save someone half an hour of their time