Add pages to doxygen

Before merge:

• enable doxygen only for main@irmb branch
• in Markdown_filter.py change path to main repo in line 46.

Link to new doxygen: https://soe.peters.gitlab-pages.rz.tu-bs.de/VirtualFluids_dev/index.html

Changes:

• adds doxygen to docker image. New docker image version: 1.2
• adds missing header to files
• unify header
• add SPDX (https://spdx.dev/) identifier to header (first step for getting REUSE https://reuse.software/ compliant)
• reworked doxgen
  • uses awesome doxygen theme: https://jothepro.github.io/doxygen-awesome-css/
  • adds wiki pages to doxygen page
  • adds doxygen modules for production code and tests

Closes #183 (closed)