I've decided to write developer documentation for Misago together with code. This documentation is not hosted on any dedicated site, but it's kept as directory with text files right next to Misago's code:
github.com/rafalp/Misago/blob/starlette/docs/index.md
So far there's only basic plugins reference, hooks reference and documentation for database. I'm hoping that documentation will be useful for plugin authors, but I have no resources to document 100% of codebase, so reading the code will still be useful when writing custom plugins I'm afraid.