Doxygen support

[TheLartians]

This PR will add automatic documentation targets using doxygen.

Current usage

cmake -Hdocumentation -Bbuild/doc
cmake --build build/doc --target GenerateDocs
# view the docs (on Mac)
open build/doc/doxygen/html/index.html

Questions

• Is Doxygen still considered modern / are better replacements available?
• Should documentation creation and/or completeness be tested in CI?
• Should the documentation target be added automatically from the main project?
  • Definitely not when used as a subdirectory, probably not by default in the template as well. That way users can simply delete the documentation directory if not needed.
• Do we need the full 2,5k line long Doxyfile?
  • Nope, we can only add the options that deviate from the default
• Can we customise the output to look more modern?
  • Not sure, using GENERATE_TREEVIEW helped a bit, but still looks quite dated

TODO

• [x] Add doxygen target
• [x] Automatically set correct project name and version
• [x] Setup doxygen to create usable docs
• [x] Add target / CI workflow for publishing documentation to GitHub pages
• [x] Add documentation to the readme

closes #40

[codecov[bot]]

Codecov Report

Merging #43 into master will not change coverage. The diff coverage is n/a.

@@            Coverage Diff            @@
##            master       #43   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files            2         2           
  Lines           12        12           
=========================================
  Hits            12        12           

Impacted Files	Coverage Δ
include/greeter.h	100.00% <ø> (ø)

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[TheLartians]

With some questions still unanswered, this is probably a good starting point for documentations and the template can be updated as more best practices become apparent.