Restructure Documentation

[MarcelKoch]

This PR splits the documentation into a part that is generated by sphinx and one that is generated by doxygen. The doxygen part will cover only the reference API documentation, and anything else will be left to sphinx. The full separation is not done in this PR.

Changes:

• adds sphinx configuration
• make doxygen available for sphinx
• simplify doxygen setup
  • only the usr doc is available, the dev doc is removed. IMO there is no need for a dev doc, since we can expect developers to look into the Ginkgo source code and discover the documentation there.
• fix some existing documentation (grouping, markdown files)
• use read-the-docs instead of GH pages to deploy documentation

TODO:

• [ ] finalize the read-the-docs namespace

[MarcelKoch]

Link to the documentation at RTD: https://ginkgo-test.readthedocs.io/en/read-the-docs/