Generate API documentation and host

[mr-smidge]

Overview

There have been at least half-reasonable efforts to document the public-facing elements of the codebase. There is also a long-forgotten Doxyfile.in in the repository root.

It should be possible to generate documentation using Doxygen (via either the meson or CMake build tools) and host this for users to see, perhaps via GitHub Pages.

Acceptance Criteria

• It must be possible for the build system to generate documentation via Doxygen.
• Generated documentation should be hosted somewhere for users to see.

[mr-smidge]

Also consider standardese as an alternative to Doxygen: https://github.com/standardese/standardese