Enable building Rust API docs

[thunderbiscuit]

I'm not sure of the best way to do this, because we need the modules to be public for the docs to build correctly, but so far they've been private (you can't use the lib from another Rust project).

This is open mainly for discussion purposes. I could also just build locally and publish. See #941

Checklists

All Submissions:

• I've signed all my commits
• I followed the contribution guidelines
• I ran cargo fmt and cargo clippy before committing
• I've added a changelog in the next release tracking issue (see example)
• I've linked the relevant upstream docs or specs above