Move documentation/ -> doc/

[dweindl]

• Use more canonical name documentation/ -> doc/
• Build doxygen documentation under doc/build_doxygen/ instead of doc/
• Make Doxyfile compatible with recent doxygen

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 77.16%. Comparing base (391c680) to head (61ef3cb).

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #2656      +/-   ##
===========================================
- Coverage    77.71%   77.16%   -0.56%     
===========================================
  Files          332      332              
  Lines        23142    23142              
  Branches      1480     1480              
===========================================
- Hits         17985    17857     -128     
- Misses        5146     5274     +128     
  Partials        11       11              

Flag	Coverage Δ
cpp	74.09% <ø> (-0.59%)	⬇️
cpp_python	33.38% <ø> (ø)
petab	39.40% <ø> (ø)
python	72.21% <ø> (-0.58%)	⬇️

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see 1 file with indirect coverage changes

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