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
Johannes Doerfert