GitLab

* Reformat restart.rst and add warning about WIP state.
* Include TODOs into user docs output.
* Include restart.rst into toctree.

* Adjust toctree to have proper sections
* Add new doc pages:
  - Feature overview
  - Data I/O
* Add new doc section 'Introduction'
* Rename main file to `index.rst`

* Do not use DUNE CMake module for creating Sphinx docs anymore
* Create subdirectories: manual, python, cookbook
* Remove unused files
* Remove 'Guided doxygen documentation'
* Add symlink from doc/manual/ to README