Sphinx documentation update

[Davknapp]

Closes #1447

Use readthedocs / sphinx to build our documentatio using sphinx and host it via readthedocs.

All these boxes must be checked by the AUTHOR before requesting review:

• The PR is small enough to be reviewed easily. If not, consider splitting up the changes in multiple PRs.
• The title starts with one of the following prefixes: Documentation:, Bugfix:, Feature:, Improvement: or Other:.
• If the PR is related to an issue, make sure to link it.
• The author made sure that, as a reviewer, he/she would check all boxes below.

All these boxes must be checked by the REVIEWERS before merging the pull request:

As a reviewer please read through all the code lines and make sure that the code is fully understood, bug free, well-documented and well-structured.

General

• The reviewer executed the new code features at least once and checked the results manually.
• The code follows the t8code coding guidelines.
• New source/header files are properly added to the CMake files.
• The code is well documented. In particular, all function declarations, structs/classes and their members have a proper doxygen documentation.
• All new algorithms and data structures are sufficiently optimal in terms of memory and runtime (If this should be merged, but there is still potential for optimization, create a new issue).

Tests

• The code is covered in an existing or new test case using Google Test.
• The code coverage of the project (reported in the CI) should not decrease. If coverage is decreased, make sure that this is reasonable and acceptable.
• Valgrind doesn't find any bugs in the new code. This script can be used to check for errors; see also this wiki article.

If the Pull request introduces code that is not covered by the github action (for example coupling with a new library):

• Should this use case be added to the github action?
• If not, does the specific use case compile and all tests pass (check manually).

Scripts and Wiki

• If a new directory with source files is added, it must be covered by the script/find_all_source_files.scp to check the indentation of these files.
• If this PR introduces a new feature, it must be covered in an example or tutorial and a Wiki article.

License

• The author added a BSD statement to doc/ (or already has one).

[sandro-elsweijer]

Looks very nice!

[sandro-elsweijer]

Suggested change

	- requirements: doc/requirements.txt
	- requirements: doc/requirements.txt

[sandro-elsweijer]

Suggested change

	breathe_projects = {"T8code": "@top_builddir@/doc/xml"}
	breathe_projects = {"t8code": "@top_builddir@/doc/xml"}

[sandro-elsweijer]

Would this call also build the documentation for all VTK/OCC specific functions?

[Davknapp]

This is a draft and currently aims to start with a minimal working example.

[sandro-elsweijer]

Suggested change

	breathe_default_project = "T8code"
	breathe_default_project = "t8code"