Consolidate docs presubmits #16625

Currently there are 2 docs presubmits:

• docs / Docs warnings: will block release
• docs / docs

The presubmit with will block release in the title failed in #16038 but is not marked as required. The second one is marked as required but it was skipped in #16038. This presubmit seems overly complicated. We should remove the presubmit docs / Docs warnings: will block release and make sure docs / docs always runs and is marked as required.