Default build 'dox' target conditional on doxygen being present

[attipaci]

This way the default 'make' does not require special build dependencies (like doxygen) being present.

Also, make help can now be used to show and explain the available build targets.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 98.79%. Comparing base (fbef9cc) to head (f4424a3).

Additional details and impacted files

@@           Coverage Diff           @@
##             main      #22   +/-   ##
=======================================
  Coverage   98.79%   98.79%           
=======================================
  Files           3        3           
  Lines        1987     1987           
  Branches      370      370           
=======================================
  Hits         1963     1963           
  Misses          7        7           
  Partials       17       17           

Flag	Coverage Δ
unittests	98.79% <ø> (ø)

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