[OpenMP] Add OpenMP Documentation for Libomptarget environment variables

Add support to the OpenMP web pages for environment variables supported
by Libomptarget and their usage.

Reviewed By: jdoerfert

Differential Revision: https://reviews.llvm.org/D93723

GitOrigin-RevId: 1c19804ebf4c97666a5c7de86ca7432c6b020205
1 file changed