Markdown link check has an issue with some links even though they work fine

[lbussell]

I have noticed that the markdown link check Github action has issues with certain links, even though the links work correctly.

• https://support.microsoft.com/contactus/ - failing build
• https://mcr.microsoft.com/product/dotnet/nightly/monitor/about - failing build

Both of the links Work On My Machine. This has been causing many valid PRs to appear as failing validation. I believe that retries have caused some of these to pass but I cannot confirm that.

Related: #5818
/cc @MichaelSimons

[lbussell]

[Triage] There are configuration options that we could use here:

1. Only enable Markdown checking on files that are modified in a PR
   • If so, then run against all files separately on a schedule
2. Increase the number of automatic retries, or retry on more/different error codes
3. Allow-list certain links to prevent them from being checked if we know they work (e.g. https://support.microsoft.com/contactus/)

[MichaelSimons]

Another issue I just noticed is 429s when validating the Docker Hub links 😞.