Add doxygen API generation into our documentation

[petejohanson]

To keep our documentation in sync, we should document all public APIs ( see #2619 ) with proper doxygen annotations, and add a doxygen step to our documentation generation to integrate proper API docs into our site.

Needs:

• Doxygen setup and generation
• Enhance docusaurus to integrate doxygen generated pages as nicely as possible (research needed here)

Optional:

• Add CI check (pre-commit? Something else?) for ensuring all public API types, functions, etc are properly documented?