Fix doxygen generation

[alexanderlinne]

Doxygen searches for relative paths like the logo relative to the working directory, not relative to the Doxyfile. We therefore set the doxygen OUTPUT_DIRECTORY to DOCS_OUTPUT_DIR call doxygen from the ./documentation directory.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 73.79%. Comparing base (c569fbb) to head (add8864).

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #442   +/-   ##
=======================================
  Coverage   73.79%   73.79%           
=======================================
  Files         259      259           
  Lines       16387    16387           
  Branches     1336     1336           
=======================================
  Hits        12092    12092           
  Misses       3878     3878           
  Partials      417      417           

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