Add zuul-sphinx as a requirement

This renames the internal sphinx domain from zuul to zuuldoc.  This
is a temporary measure to avoid colliding with the domain defined
in zuul-sphinx (because of the initial data required, it's not
easy for us to simply add new directives/roles without at least
porting *some* of the framework into zuul-sphinx).

I expect to do that when this has stabilized.

Change-Id: I26be542a6d4f6266f7843ada5939172656b8b847
Depends-On: I8a1534f7c2614ee11411cf228de38931257fc970
3 files changed