[DOC] Implement link checking with `lychee` in CI

[jcrist]

The goal here is to use lychee in CI (e.g. lychee GHA) to validate links in our repo. We want to run this periodically in a cron job (but not on every PR), and add it to the "recently failed" check in CI to surface any errors.

xref: rapidsai/build-planning#9