Add documentation pages

The current documentation via the Readme is fine, but fairly long, and somewhat incomplete.

This setup should be moved to Documenter.jl, also rebuilding the performance plots when deploying on master, but by default not on MRs.

See https://stackoverflow.com/questions/36191766/metaprogramming-within-docstring-with-eval for adding DocStrings.

Start by reviewing the current README and the requested Documenter.jl migration, then inspect how performance plots are generated and how master builds differ from merge requests. Use the linked metaprogramming reference when evaluating docstrings. Done means the documentation is separated from the README and deployment rebuilds plots only on master.

Vom Indexierungsmodell aus dem Issue-Text verfasst.