Generalize doc build source

A continuing quest to streamline building of ReST pages in a Github world.

The major part of this change is to move all project specific information to a different file that is included.  That will mean that this Makefile can be just copied to other projects as is, and only the `gh-project.mk` file needs to be updated/established for each.

I hope that this will work seamlessly for gh-pages branch-based sites as well as <account>.github.io sites.

This PR should be applied to `source` and a matching one will be applied to `master`.