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/faq/faq.md

@@ -1,3 +1,5 @@
+(faq)=
+
 # Frequently asked questions
 
 ## How do I share links to notebooks?

docs/source/faq/institutional-faq.md

@@ -1,3 +1,5 @@
+(faq:institutional)=
+
 # Institutional FAQ
 
 This page contains common questions from users of JupyterHub,
@@ -130,7 +132,7 @@ level for several years, and makes a number of "default" security decisions that
 users.
 
 - For security considerations in the base JupyterHub application,
-  [see the JupyterHub security page](web-security).
+  [see the JupyterHub security page](explanation:security).
 - For security considerations when deploying JupyterHub on Kubernetes, see the
   [JupyterHub on Kubernetes security page](https://z2jh.jupyter.org/en/latest/security.html).
 

docs/source/faq/troubleshooting.md

@@ -1,4 +1,4 @@
-(troubleshooting)=
+(faq:troubleshooting)=
 
 # Troubleshooting