Add documentation on using libdave

[onerandomusername]

We should build this with either sphinx or mkdocs. I'm almost inclined to use mkdocs for this project.

[shiftinv]

Something to note for documentation specifics - we should especially be careful with documenting object ownership, i.e. explicitly mention that ownership is transferred to C++ for objects passed to methods that take std::unique_ptrs, where applicable.

(fwiw, this isn't a security issue, since nanobind invalidates the objects on the Python side when transferred to C++ and throws when reused, it's just a potential pitfall)