CI: issues with linkcheck for GH links

[bsipocz]

GH feels to be rather flaky recently and we see a lot of 429, Too Many Requests errors for file download and linkchecks (not just in our repos but it came up as an issue in other github organizations).

(https://github.com/Caltech-IPAC/irsa-tutorials/actions/runs/22844039330/job/66482642714#step:5:3048)
  - Link for "https://github.com/Caltech-IPAC/jupyter_firefly_extensions/blob/master/README.md" did not resolve. (429, Too Many Requests)
  - Link for "https://github.com/Caltech-IPAC/jupyter_firefly_extensions/blob/master/README.md" did not resolve. (429, Too Many Requests)
  - Link for "https://github.com/Caltech-IPAC/firefly/blob/dev/README.md" did not resolve. (429, Too Many Requests)

I don't think we can do much about this right now, but nevertheless I open this issue so others see more easily the current situation.

(I will report back if we hear back from github and if/when they tell us if this is a temporarily issue or a more permanent change).

[bsipocz]

Apparently this is due to a recently shipped "feature" rather than a bug, so I'm adding a temporarily workaround for us and will wait and see if an actual solution appears.

https://github.blog/changelog/2025-05-08-updated-rate-limits-for-unauthenticated-requests/

[bsipocz]

Also, workaround, which may actually work as most of the failing links are firefly docs cross links --> we should link to their deployed docs rather than to the github repo readme.

I have previously encountered this issue with conda-smithy linter PRs and with other JSON files commonly downloaded as part of docs/build workflows. I do not have a solution for this, of course; however, if you are looking for a (reasonably) reliable workaround, one is to use GitHub Pages to host such files and query their URLs, which won't trigger these stringent rate limits.