Feature/doxygen markdown main readme

• Modify actual readme to look the same in GitLab and doxygen
• Improve sections What is Dumux
• Fix links
• Create anchors with filter
• Resolve installation section in readme and put in Documentation
• rename Documentation in README to Overview
• modify script such that all .md are processed (runtime flag for header-level modification used in readme). Call math-modification script internally. The goal is that all headers in all .md files get anchors
• Fix links to papers (seems to be too complex of a markdown link for doxygen)
• Write compatibility table in htmlmarkdown

Concept:

• Don't add default "pages" in the doxygen layout file
• Add pages and subpages manually into the sidebar
• TOCs in markdown are created with the [TOC] command

This allows to

• Split the module description into several files
• The use of @addtogroup to combine module documentation