Make rocm_add_sphinx_doc OUTPUT_DIR relative to the source directory not build directory

[pfultz2]

This broke deploying docs to github pages. We dont want to add the build directory to git for github pages nor do we want to add the build directory when distributing tarballs with the source and prebuilt documentation.

Documentation is meant to go into the source directory. That is why make clean will clear these directories so they can be removed for the cases where you dont want to commit them or distribute them.