GitLab

* Moved `COPYING.md`, `CODE_OF_CONDUCT.md`, `CONTRIBUTING.md`, and `LICENSE` (download only) into user manual #177.
* Fixed formatting of GMSH command #177.
* Fixed various links in existing documentation.
* Updated link to user manual in CLI.
* Added hints on cook book to `README.md` and user manual index.
* Fix render of `README.md` tables in user manual.

Set default output paths and file names for both models

Manual:
* Update links to removed `.bcdat` files.
* Fix link in "Restart" cook book page.

Description Templates:
* Fix link to `CHANGELOG.md`.
* Add hint for MR assignee to check the squash commit message.

* Move into folder '1-infiltration-sand'.
* Remove trailing whitespace from files.
* Add captions to source code.
* Slightly reformulate instructions.
* Add more links to manual.

* 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

Signed-off-by: Santiago Ospina <santiago.ospina@iup.uni-heidelberg.de>

Also remove unnecessary mentions of 'DORiE' in document titles.