Jeckyll redirect_from incompatible with HTMLProofer url validation

Renaming a URL and using the Jekyll redirect_from feature breaks pre-commit validation.

We hit this on PR580:

The scenario for using a redirect is if you're moving pageA.md to pageB.md; you would rename the file to pageB.md and add redirect_from: pageA. This will cause jekyll to generate a pageA.html with a redirect to https://beam.apache.org/pageB.html. But pageB.html doesn't exist on the livesite yet, so it will always fail.