[OpenMP][Docs] Structure and content for the OpenMP documentation

This adds some initial content as well as structure to the new OpenMP
Sphinx documentation hosted at http://openmp.llvm.org/docs/ .

The content contains some useful links but most pages are still empty.

This uses a "custom" theme which is a copy of the default "agogo" one
with minor modifications to get a nicer table of content in the sidebar.
This way we can also adjust the theme as we go.

Reviewed By: jhuber6, JonChesterfield

Differential Revision: https://reviews.llvm.org/D90256