Add documentation for using Doxygen

- Add a page that provides guidelines and examples for using Doxygen.
  Hopefully, this would make simple examples easier to find, and help
  people write correct comments from the beginning.
- Remove unnecessary \addtopublicapi and \addtolibraryapi.

Once the contents are there, it is easy to enhance the documentation by
adding content whenever noticing things that are tricky.

Change-Id: I319e30452004998df73c249f46570cb2254daa87