Add docs to CI

[jturner314]

This is primarily to detect broken intra-doc links, which cargo doc warns about.

This should be rebased off of master once #924 is merged to make sure the existing warnings (when the docs feature is not enabled) are treated as errors. Then, we can change the command to cargo doc --features=docs to resolve the warnings.

Edit: It looks like this is blocked on rust-lang/rust#79816 in order for -D warnings to apply to rustdoc.