- updated the doxyfile using doxygen -u
- include hidtest/test.c as a doc'd example
- use a brief doxygen/main_page.md file as the generated site's main page
- strip absolute paths from doxygen output (which now displays as relative paths to repo root)
- add CI to build the docs and save as a CI artifact and rewrite the contents of the gh-pages branch for deployment (for master builds)
- add some missing docstrings and excluded undocumented "helper macros" to satisfy warnings output in doxygen's build log
