The djblets.cache documentation was amongst the oldest sets of docs in
the codebase, and weren't anywhere close to PEP-257-compliant, nor did they
render well in our new doc builds.
I've gone through and updated the public-facing documentation, making it
compliant with PEP 257 and helping it render links and text properly.
I've also fleshed out some of the documentation to clear up some points.
In order to link to Django docs for some of the settings, I needed to
add a new extension, djangorefs, which sets up the cross reference types
needed.
