resolves #5

• 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)
• added CI to build the docs and save as a CI artifact (useful for visual inspection without checking out a PR branch). This new CI will also rewrite the contents of the gh-pages branch for deployment (only for push events to the master branch).
• added some missing docstrings and excluded undocumented "helper macros" to satisfy warnings output in doxygen's build log.

Admin prep

The required preparation needed from admins is

1. make sure the repo's setting are set to use the gh-pages branch root folder as source of deployments.
   image
   I realize that there is already a branch named github_pages, but I think that branch was meant for a different purpose. The branch published to github pages typically only has the site's HTML resources (not any programming or documentation sources) -- especially when using Doxygen as the static site generator.
2. make sure the custom domain is configured correctly. I mention this one because https://libusb.github.io/hidapi forwards to https://libusb.info/hidapi
3. add the deployed site's link to the repo's "About" settings:
   image
   Alternatively (or additionally), a link could be added to the README in the "What does the API look like?" section.