add cpp api reference (#618)

Summary:
Pull Request resolved: https://github.com/pytorch/executorch/pull/618

Adding cpp docs and updating the build system for the docs to support doxygen and breath (connects sphinx and doxygen).

If anyone in the future wants to add similar cpp docs they can just reference their cpp classes and methods like we do in executorch-runtime-api-reference.rst, and then add the corresponding .h files to Doxyfile's INPUT arg.

Reviewed By: svekars, cccclai

Differential Revision: D49924199

fbshipit-source-id: 6abc26166158d5ab9ff70f6b9904269ec9d62489