Update dynamic README workflow to open a Pull Request

[stefannibrasil]

Some repositories have a protected branch rule. That means our workflow won't work because commiting directly to main is not allowed. To prevent that, the workflow will now open a PR for the changes. Rather than chancing the repo rules, updating the workflow is easier. Probably safer, too.

Other updates include only running the workflow when the README is updated. And removing the leading / that indicates it's a file (but it isn't). @nickcharlton pointed it out here. It must have been needed when I was testing things but it works fine without it on this test repo.