Online Documentation with GitLab pages

This MR enables the online documentation of mHM based on the present Doxgen documentation:

• switch to select the documented branch (master/develop/v5.11/...) was added to the webpage to switch between versions of mHM.
• the documentation can later be accessed by: https://mhm.pages.ufz.de/mhm
• by default, the documentation will point to the master version (which is not present yet)
• test version can be accessed by: https://muellese.pages.ufz.de/mhm/doc_update (you can only switch to develop there, since master is not present)
• the lib folder was excluded in the documentation, since it will get its own one
• the documentation will be build automatically during CI runs in the build stage
• along with the online documentation, a pdf version will be build and provided as an artifact by the Gitlab-runner