Fixes documentation intra-links and enforce it

[RCasatta]

This fixes documentation intra-links so that are correctly rendered in generated HTML pages and on docs.rs

It also adds #![deny(broken_intra_doc_links)] and build docs in CI so that future errors of this kind are seen by the CI

Do we want to use nightly to build the docs since it's the version used on docs.rs?

[sgeisler]

utACK c96f7ec

[dr-orlovsky]

LGTM.

As a side note, from what I know, its common to use [`...`] notation for referencing types & fields, rather than simple [..]

[apoelstra]

sure