doc: libnetplan API reference, using Doxygen and Sphinx.breathe

[slyon]

Description

Setting up a structure to make use of Doxygen and Sphinx' Breathe the generate a API documentation reference for libnetplan directly from the source and header files.

The documentation is generated at doc-build time (make run inside doc/) and also run on readthedocs, so we show the most current documentation from the sources at all times.

We make use of Breathe's autodoxygen features, so that we don't need to generate and parse Doxygen's XML output manually.

FR-3781

Checklist

• Runs make check successfully.
• Retains 100% code coverage (make check-coverage).
• New/changed keys in YAML format are documented.
• (Optional) Adds example YAML for new feature.
• (Optional) Closes an open bug in Launchpad.

[slyon]

We might even consider dropping the top-level Doxyfile configuration, I think it's not necessarily needed.

[rkratky]

@slyon A few nitpicks.

[slyon]

Thanks a lot for the prompt review! I addressed all the nitpicks. Will merge once the CI is (mostly) green.