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/howto/configuration/config-ghoauth.md

@@ -1,3 +1,5 @@
+(howto:config:gh-oauth)=
+
 # Configure GitHub OAuth
 
 In this example, we show a configuration file for a fairly standard JupyterHub

docs/source/howto/configuration/config-proxy.md

@@ -1,3 +1,5 @@
+(howto:config:reverse-proxy)=
+
 # Using a reverse proxy
 
 In the following example, we show configuration files for a JupyterHub server

docs/source/howto/configuration/config-sudo.md

@@ -1,3 +1,5 @@
+(howto:config:no-sudo)=
+
 # Run JupyterHub without root privileges using `sudo`
 
 **Note:** Setting up `sudo` permissions involves many pieces of system

docs/source/howto/configuration/config-user-env.md

@@ -1,3 +1,5 @@
+(howto:config:user-env)=
+
 # Configuring user environments
 
 To deploy JupyterHub means you are providing Jupyter notebook environments for