Doxygen + GitHub Action

[salehjg]

Implements:

• Doxygen target.
  • Graphwiz-dot support.
  • plantuml support.
  • CMake modules to search for dot and plantuml.
  • Nice-to-have class-hierarchy diagrams like the ones on my fork.
  • User can build the Doxygen target (it's called doc) offline as well.
  • Two Doxygen config files, one for the offline build (Doxygen.in) and another for the Action build (DoxyfileGithubAction).
• GitHub Action to build and deploy the Doxygen target on the repo's GitHub Page.
  • The action needs R/W access to the repo.
  • Creates a separate branch called gh-pages for the HTML files ...
  • The repo should be configured to deploy its Page on the mentioned gh-pages branch.

[satya-das]

It will be a nice addition to the cppparser. Please start this PR again with the latest code. The code has been refactored a lot. Mainly to improve its quality and robustness. In future there will not be any large disruptive refactoring.