[HOLD] Spike: link-checking with excluded versions

[pepopowitz]

DO NOT MERGE!!!

Part of #1659

Questions answered

1. ❌ Will a link-check of a build that doesn't include all versions pass?
   No, it will not pass. Here is a build of only the latest versions, and here is a build of only older versions. (see commits 1 and 2 in this PR.)
2. ❌ If I reduce the sitemap to be just the versions included, will that pass?
   Not likely, because we also check product-links.txt, and that also includes a bunch of URLs.
   It might pass if the build includes the current version, too.
3. ✅ Will a link-check pass if the build includes only the current version and affected versions, and the sitemap links are limited to the current/affected versions?
   This workflow demonstrates that a build that includes the next, current, and one old version will pass when it is link-checked for only the next, current, and one old version URLs. (see commits 3 and 4 in this PR.)
4. ✅ Why are cron-based link-checks failing regularly? If these were more stable I'd feel more comfortable about doing partial link-checking on PRs.
   Lots of connection errors: see this one and that one.
   There is actually a flag we can pass to the link-checking tool to report connection failures as warnings instead of errors. I've PR'ed a change for this at ci(linkcheck): use github action instead of downloading tar, and ignore connection failures for cron-based workflow #1676.

[pepopowitz]

Closing this as it was for experimentation only.