docs(reference): update client reference link

The client documentation should handle an internal link to client,
while the client reference should also get its own. There are no
known links to the client reference docs, so no other changes are
necessary at this time.
docs(reference): update reference guide
docs(reference): use deis <command> as title

This makes it such that when you reference the command in the
documentation as ":ref:`deis_releases`", it shows up as "deis releases".
ref(docs/reference): aggregate client commands
style(docs/reference): missing period
Merge pull request #1327 from deis/fix-reference-guide

fix(docs/reference): the big reference guide refactor