Updates to 404 checker in PR workflow

[thoward]

Currently we have a 404 checker that looks for new broken links/missing pages, which happens before merge during the PR workflow. If this fails, it will block the merge of a PR. Unfortunately, while this tool often reports correctly, it also often reports incorrectly, causing problematic merge workflows. For that reason, we have temporarily disabled it in PR #13775 .

This task would be to update that workflow such that:

• 404 checker is re-enabled
• it does not block PR merge
• instead, it runs similarly to our site previews (pre-merge) and reports results into the PR as a comment

With these changes, authors of PRs can get a signal from the 404 checker without it interrupting the workflow. An author can inspect each reported 404 link and fix any that are real issues, and ignore the ones that are false reports.

Later, we will invest in improving the accuracy of the checker and possibly restore it to a blocking part of the PR workflow, but these changes will allow us to get value from it without it interrupting productivity.

[ericrudder]

i think we should keep the behavior that blocks the PR merge .., that's the main idea of a hygiene gate like this.

let's make sure to get this re-enabled ASAP tho.