Remove Doxyfile from root of open-amp directory in favor of Doxyfile.in

[tammyleino]

libmetal uses the Doxyfile.in file in the doc directory. OpenAMP should be updated to operate the same way instead of using the Doxyfile at the root. This will also require changes to build the documentation using cmake instead of Doxygen. Also, add a README to the doc directory.

[github-actions]

This issue has been marked as a stale issue because it has been open (more than) 45 days with no activity.