Repo documentation

For ragconnect developers, it would be nice to have some documentation attached to the code.

• A link to the actual documentation (connector.relational-rags.eu). Either in the project description or in a README
• version tags in the repo
• switch from sphinx to mkdocs (easier to handle)

@rschoene feel free to add/edit/close this issue. These were just the things I missed when following up on a preprocessor-related bug.