Update writing-an-authenticator documentation to a tutorial

Currently we have a how-to section called writing-an-authenticator.md accompanied with a python file example-oauthenticator.py

I think we should refactor this like...

[ ] extract reference docs from example-oauthenticator.py It describes a lot of the functions, but that kind of content can make sense to have in the docstrings of the functions we override, so lets see if we can extract things of relevance to the function docstrings.
[ ] relocate the how-to document to the tutorial section The tutorial section is learning oriented, the how-to section is task oriented. I figure this is more learning oriented because we can't reasonably help the user arrive all the way anyhow.
[ ] let the tutorial rely on this not yet written explanation section: https://github.com/jupyterhub/oauthenticator/issues/634
[ ] let the tutorial rely on linking to reference documentation
[ ] let the tutorial be something that ties this together rather

jupyterhub / oauthenticator