deploy docs using gh-pages

[2bndy5]

Introduces a new workflow that:

• only runs on master branch
• only deploys docs on release "publish" or "edit" events
• fetches latest version tag from this repo's releases and uses it to update the Doxyfile's "PROJECT_NUMBER" tag. This tag change is not committed to the Doxyfile, rather it is only used during the workflow.

Additionally, I've

• made fixes for all "addressable" doxygen warnings (Makefile is parsed like a C source file because there is no extension -- its a doxygen problem)
  • This includes removing deprecated tags from the Doxyfile (they were using default or invalid options anyway). I didn't use doxygen -u for backward compatibility reasons.
• formatted and updated the doxygen-custom.css file
• renamed all links to docs from tmrh20.github.io/<repo name> to nRF24.github.io/<repo name> except for:
  • the links to the automatic install/update script at tmrh20's RF24Installer for RPi
  • any links that lead to tmrh20.github.io main page listing all RF24 repos (links for this page have to be changed separately in the tmrh20.github.io repo's index.html file
• deactivated any LATEX or XML output from doxygen since they're not actually being used (and the LATEX output seems to require 3rd party software to generate properly).