[OpenMP] Initial Support for OpenMP Webpage Documentation

Summary:
Adding support for generated html documentation for OpenMP. Changing
Cmake files to build the documentation and adding the base templates for
future documentation to be added.

Reviewers: jdoerfert

Subscribers: aaron.ballman arphaman guansong mgorny openmp-commits sstefan1 yaxunl

Tags: #OpenMP

Differential Revision: https://reviews.llvm.org/D87797