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/explanation/capacity-planning.md

@@ -1,3 +1,5 @@
+(explanation:capacity-planning)=
+
 # Capacity planning
 
 General capacity planning advice for JupyterHub is hard to give,

docs/source/explanation/concepts.md

@@ -1,3 +1,5 @@
+(explanation:concepts)=
+
 # JupyterHub: A conceptual overview
 
 ```{warning}
@@ -201,7 +203,7 @@ running, the notebooks). By default, the hub starts the proxy
 automatically
 and stops the proxy when the hub stops (so that connections get
 interrupted). But when you [configure the proxy to run
-separately](separate-proxy),
+separately](howto:separate-proxy),
 user's connections will continue to work even without the hub.
 
 The default proxy is **ConfigurableHttpProxy** which is simple but

docs/source/explanation/database.md

@@ -1,4 +1,4 @@
-(hub-database)=
+(explanation:hub-database)=
 
 # The Hub's Database
 

docs/source/explanation/index.md

@@ -1,3 +1,5 @@
+(explanation)=
+
 # Explanation
 
 _Explanation_ documentation provide big-picture descriptions of how JupyterHub works. This section is meant to build your understanding of particular topics.

docs/source/explanation/oauth.md

@@ -1,4 +1,4 @@
-(jupyterhub-oauth)=
+(explanation:hub-oauth)=
 
 # JupyterHub and OAuth
 

docs/source/explanation/singleuser.md

@@ -1,4 +1,4 @@
-(singleuser)=
+(explanation:singleuser)=
 
 # The JupyterHub single-user server
 

docs/source/explanation/websecurity.md

@@ -1,4 +1,4 @@
-(web-security)=
+(explanation:security)=
 
 # Security Overview