Provide consistent myst references to documentation pages

While doing https://github.com/jupyterhub/jupyterhub/pull/2726,
I realized we don't have a consistent way to format references
inside the docs. I now have them be formatted to match the name
of the file, but using `:` to separate them instead of `/` or `-`.
`/` makes it ambiguous when using with markdown link syntax, as
it could be a reference or a file. And using `-` is ambiguous, as
that can be the name of the file itself.

This PR does about half, I can do the other half later (unless
someone else does).

docs/source/reference/api/index.md

@@ -11,7 +11,7 @@
 :Release: {{ version }}
 
 JupyterHub also provides a REST API for administration of the Hub and users.
-The documentation on [Using JupyterHub's REST API](using-jupyterhub-rest-api) provides
+The documentation on [Using JupyterHub's REST API](howto:rest-api) provides
 information on:
 
 - what you can do with the API