Rust Docs CI

[plebhash]

as @Shourya742 pointed out, there's a very interesting rustdoc lint macro available: missing_docs

by combining it with the deny filter, we have a powerful filter that if integrated into SRI CI, will make sure we never let any public API go without docs ever again.

this will give us confidence to move forward from #845 knowing that we didn't let anything go without notice

some files already contain this, but we need a PR that adds this macro to every file that still doesn't have it.

#![deny(missing_docs)]

this PR should also add a new CI workflow that enforces running cargo doc --all-features recursively across all crates in the repo