Fix broken links

In #2820, we added a workflow that shows all the broken links in this repository and, most likely, on our website as well. Ideally, we'll add this workflow to PRs, but first, we need to fix the existing broken ones.

The existing workflow only tells us where the broken links are; now we need to perform an assessment and decide on a case-by-case scenario if:

• We should add an exception because it's failing locally but not on the website
• The link should be replaced by a more up-to-date link
• The link doesn't exist, there's no replacement, and therefore, we should use a link from the Internet Archive.

I'm still not sure how to organize this work, but I can see us breaking this whole problem into multiple smaller tasks. Let's discuss how to solve the problem before opening PRs :)

Start with the broken-link workflow introduced in #2820 and review its reported results for this repository and website. Assess each link as a local exception, an updated replacement, or an Internet Archive link; the work is done when the existing broken links have been classified and addressed, but the issue does not yet define how to split the work.