What does this implement/fix? Explain your changes.

Adds Sphinx documentation using doxygen and breathe.

Does this close any currently open issues?

Among general documentation, will include #1406

TODO:

• Implement doyxgen/breathe/sphinx setup
• Update the included Embedding documentation to reflect the current state (esp. regarding installation)
• Ensure that the (Python) code examples still work (i.e. make them doctests)
• Separate installation documentation from the embedding sections, update and maybe add distribution-specific instructions (Add Archlinux Dependencies #1625).
• Add Python reference and link clr-loader documentation (intersphinx)