Link validation doesn't catch GH link changes in the current PR

Readmes are checked for broken links using Verify-Links.ps1. That script checks that the URLs in a markdown or html file are valid and don't return an error result, e.g. 404.

The current script doesn't detect that a PR will be deleting a file that a url targets. It also doesn't detect that a change creates a file that a url targets when the github org isn't `Azure`.

We should improve the script so that:
- validation fails if the current PR deletes a file that a url references (when the url targets main, etc)
- validation succeeds if the current PR creates a file that a url references (even when the github org != 'azure')