Add CI to check for dead links in docs

[alice-i-cecile]

What problem does this solve or what need does it fill?

cargo doc doesn't seem to emit errors if you point it to the wrong location in your docs link. This is going to be particularly painful to catch during large refactors of code structure.

This is also relevant for any external links we may use.

What solution would you like?

Add a task to the CI to ensure that all links within the docs point to a valid location.

What alternative(s) have you considered?

Wait for users to complain that the links are broken T_T

File a bug with cargo doc or find a way to configure it to show errors on dead links.

Additional context

Any other information you would like to add such as related previous work,
screenshots, benchmarks, etc.

[alice-i-cecile]

Apparently there's a nice easy way to do this: https://github.com/deadlinks/cargo-deadlinks/