Add Doxygen documentation generator workflow

[Lathreas]

This pull request adds a GitHub workflow to automatically generate pretty documentation.

It adds three files:

• The documentation.yaml file which tells GitHub that it should regenerate the documentation each time a change occurs in the master branch;
• A Doxyfile, which tells Doxygen how to generate the documentation, and
• doxygen-awesome.css, which is a style sheet to make the documentation look pretty, taken from the doxygen-awesome repository. It was included directly in the working tree and not as a submodule because Doxygen couldn't find the css file otherwise. Let me know if that's okay, or if it'd be wise to spend more time trying to get it working as a submodule instead.

The documentation will look something like this (although with more actual text once the raytracing branch is merged): https://lathreas.github.io/anatomy3d/

[Lathreas]

This one should be squashed and merged, since it only really adds one thing.

[AdamNorberg]

LGTM