I mostly use doxygen (and javadoc sometimes) for most of my projects.
Rust has its own documenting subsystem.
How can I mix them for projects which use both Rust and C++/Python/... parts?
What I want is mixed /docs/ documenting bundle in HTML (available for reading via GitHub pages), which is compatible with Rust package publishing policies.
PS: maybe you can advise some manual on writing and packaging mixed-language libraries in Rust + C++ where this problem is covered?
I don't have advice on how to integrate doxygen (or if it is possible), but regarding "compatible with Rust package publishing policies" — you can provide an arbitrary URL for your package's documentation. (If you don't provide that URL, there's automatically a link to docs.rs, but docs.rs purely runs rustdoc on uploaded packages, not using any prebuilt documentation, so it's not suitable for your use case.)