Add docs for Hertz structs

[jamwaffles]

Adds docs for the various time structs. I'm using the nightly doc link syntax as it's easier to throw links to stuff everywhere. Build the docs with cargo +nightly doc to see them working. docs.rs already does this so they should render correctly there.

This PR also adds #![deny(intra_doc_link_resolution_failure)] for future use. It's a noop in stable but helps with link checking in nightly. Happy to remove if people don't want it in yet.

TODO

• Add docs for Hertz
• Add docs for KiloHertz
• Add docs for MegaHertz

Cc #158

[therealprof]

LGTM, thanks.