Implement link checking workflow

[matthiasrohmer]

Part of #45. Implement a link checking workflow, that checks for broken links on a page. Ideally the tool only runs on the changed page in a PR (to safe time and prevent positives on files unrelated to the PR) but is also able to check the full site - ideally not statically but served via our own express app.

As mentioned in #45, the problem spans wider than just broken links: we also want to make sure every page we generate is also discoverable (say linked, see GoogleChrome/developer.chrome.com#83) on the site and we also want to make sure our external sources are not broken.

• Start a product requirements doc (PRD), evaluating and documenting possible existing tools and their pros/cons (https://github.com/untitaker/hyperlink, https://github.com/lycheeverse/lychee, ...) with the goals mentioned above in mind
• Implement the checking logic in this repo (for example as a gulp task)
• Add task as a workflow to d.c.c
• Add task as a workflow to web.dev
• Fix existing broken links

[mollietokyo]

Estimate for dev = 8 days (SPR-3)

[Blazzike]

awaiting PR review: GoogleChrome/developer.chrome.com#5667

[mollietokyo]

spoke with Matthias, he's waiting on internal updates prior to marking this one as 'done'