All published crates should have rustdoc tested in CI

[nagisa]

If we're publishing the crates, they are naturally being used by somebody and their documentation is being read as well. docs.rs by default is pretty conservative, but one thing it does is generate documentation using a reasonably recent nightly. We should have a reasonably-similar check in our CI to ensure documentation generation continues to work.

[nagisa]

(There are a couple of ways doc generation can start failing: lints, nightly changes making things not compile on nightly anymore, weird dependency situations, the fact that all features are enabled all at once...)