Test that cargo doc runs successfully in CI

[Anders429]

We should check that cargo doc runs successfully in CI, promoting all warnings to errors using the RUSTDOCFLAGS='-D warnings' env variable (see this issue for discussion on using this env variable). Ideally, we should also attempt to build documentation in as close to the same way as what docs.rs will be doing. This should hopefully prevent having to republish due to failures occurring in docs.rs generating documentation.