Migrate user manual to MkDocs

[buhtz]

This is a meta issue to coordinate and document the steps migrating our documentation to MkDocs.

• Initial MkDocs setup incl. the user manual content (PR doc: Manual tests and installing #1837)
• (in progress...) Document build dependencies to generate the doc.
• (in progress...) Update doc-dev documents regarding MkDocs.
• Add user-callback docu (Issue User manual: Document user-callback scripts for users (migrate to MkDocs) #1659)
• (in progress...) Setup ReadTheDocs to use this MkDocs.
• Decide (remove?) about "user-callback" repo and its ReadTheDocs instance.
• Remove "doc" repo.

HowTo generate doc on your local system using MkDocs

To generate the doc on your local system, navigate into /doc/manual. Now you have to options: 1) Run mkdocs build to generate HTML and open /doc/manual/html/index.html, or 2) run mkdocs serve and start your browser with http://127.0.0.1:8000 ("localhost:8000" doesn't work on my system). You can modify the config and the content (md-files) life while the server runs.