[CI] add doxygen build job

It would be nice if the doxygen documentation was built in the CI and made available via artifacts. Then, one can look at the docu of newly added classes/functions to check if they look good.

Since we don't have to do this in each test pipeline, it should be a separate job. The same could be done with codespell/linting. We could collect everything in one stage with all jobs executed in parallel:

• linting
• codespell
• docu
• test pipeline triggers

EDIT: flake8 seems to require an installation of the python bindings and must therefore be after configuration, thus, within the test pipeline(s)