PR validation of documentation is lacking

[theunrepentantgeek]

Describe the bug

We managed to merge PR #4261 without catching a syntax error in a relref expression.

Expected behavior

Our CI checks should have caught it.

[theunrepentantgeek]

Catch 22:
If we make the Validate Documentation Links action mandatory, we can end up blocked when a PR both adds new pages to our docs and references those pages.
If we retain Validate Documentation Links as optional (the current state), then we can merge broken documentation changes.

Since these are easy to fix, we'll retain the current stage, but with a reminder to all core contributors to check the status of this action explicitly before hitting merge.