Generate documentation

[Shillaker]

Changes:

• Add Doxygen doc generation for C++ code
• Integrate with Sphinx using Breathe
• Add full docs generation to Github Actions build

Docs are published at https://faabric.readthedocs.io/en/latest/

Note that the RTD build runs on the RTD servers, so the wrapper used in the GHA build is just to make sure it doesn't error, it doesn't actually produce the final result.

[Shillaker]

I'll remove these TEMP steps when we do the next release.