doxygen docu build in pipeline & deploy to gitlab pages

Description

( Summarise the changes introduced in this merge request )

Check before assigning to a maintainer for review

• the history is clean
• code is well documented and understandable
• there is a testcase (in case of new functionality)
• every commit passes the pipeline

Maintainer checks before merge

• Review is approved (and all comments are resolved)
• Check for Testsuite changes, if yes check
  • Testsuite merge request merges fast-forward
  • Testsuite-submodule SHA of every CFS commit references a commit in Testsuite Merge Request
  • Testsuite-submodule SHA of last CFS commit points to HEAD of corresponding Testsuite branch
• Pipeline passes for every commit
  • all stable tests pass
  • testsuite tests run
  • new tests are actually running (e.g. check if they appear on CDash)