[prdoc] Require SemVer bump level (#3816)
A prerequisite for adding a stable branch and respecting SemVer on new stable releases is including SemVer bump levels in our PRDocs. Next release is scheduled for April 3rd, so it would be great to get this merged before then. Also added "None" as a valid bump option, to support test/benchmark changes and CI to ensure changed crates have an entry.