should CI run with `-Dmissing-docs`

[cbeck88]

Usually I put #![deny(missing_docs)] in all my crate's lib.rs, and I'd like to encourage other people to do this.

It occurs to me that maybe I shouldn't put this in every crate's source code, and just put it in CI instead

[jcape]

I think putting it in both is ideal. CI forces the issue, but every crate should have that deny as well. In the current templates we deny that, unsafe, and missing debug