Migrate user manual to MkDocs

[buhtz]

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

• [X] Initial MkDocs setup incl. the user manual content (PR #1837)
• [X] Document build dependencies to generate the doc.
• [X] Update doc-dev documents regarding MkDocs.
• [ ] Add user-callback docu (Issue #1659)
• [ ] Consider open issues in "user-callback" repo
• [X] Setup ReadTheDocs to use this MkDocs.
• [ ] Mark "user-callback" repo as deprecated and "Public archive".
• [X] Remove "doc" repo. Mark "doc" repo as deprecated and "Public archive".

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.

[buhtz]

The user manual now is build with MkDocs by Read The Docs. The repo bit-team/doc now has a deprecated message in the beginning of it README.md and I set the repo into "archive" mode.

[buhtz]

https://mail.python.org/archives/list/bit-dev@python.org/message/JDU4EYFYKFA6YZZD2WP2WKPE4QA5UIBA/