Refactor documentation Danger plugin and add specs; fix ~type fighting

What does this MR do and why?

• Refactors the documentation Dangerfile into a plugin
• Adds reasonably extensive specs
• Fixes documentation/Dangerfile fights with you over ~... (#581840) — don't add typemaintenance if there's already a ~type label, as docs changes alongside e.g. a feature or bugfix shouldn't change the MR type from that.

References

See also Danger: Ensure that Dangerfiles have test coverage (#415450).

How to set up and validate locally

bundle exec rake danger_local runs Danger locally; you can try making docs changes on top of this branch, commit, and then run that to see the output.

MR acceptance checklist

Evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.