Turn on deny missing docs

In https://github.com/googlefonts/fontmake-rs/pull/79#discussion_r1066273118 @cmyr noted:

> root lib should have //! at least some module comment. This is the text on the root docs page for this crate.
>
> Also, I think it's worth turning on deny(missing_docs); it can be annoying, but it has real benefits, and it's even more annoying to turn on in three months and then have 300 errors to fix. You can always judiciously use #[allow(missing_docs)] on specific items where you don't think docs are useful, like large enums.