[cmd/chloggen] Update CI process

[djaglowski]

The process should work as follows:

• Require that CHANGELOG.md is unchanged
• Require one new .yaml file in ./unreleased/
• Require zero modified, deleted, etc changes in ./unreleased/
• Require that make chlog-validate passes
• Render the new release section of the changelog and run markdown-link-check against it (not a skippable step)
• If any of the above fail, explain where to read about new process