At some point in December 2024 I decided to move documentatation from markdown files to Doxygen but I have not yet found the time to host the result somewhere on the internet.
So for now you can find high-level non-API documentation in .dox files.
Or, if you feel proactive, you can build the full documentation locally via the docs CMake target,
for examplecmake --build build --target docs and you should get a build/docs/html/index.html file
that you can open with your favourite browser.