Consider cleaning up extra markdown documents

[domenic]

• docs/urlpattern.md: it's basically a redirect

• mdn-drafts/QUICK-REFERENCE.md: subsumed by actual MDN docs, I think? https://developer.mozilla.org/en-US/docs/Web/API/URLPattern

• 202012-update.md, dev-trials-feedback.md, howto.md: these seem like artifacts from the development process and/or Chromium Origin Trial process, which probably don't need to be maintained forever.

• tag-security-privacy.md: TAG review has concluded, and this contains a lot of service-worker specific stuff

Note that I think keeping explainer.md is reasonable, or at least deleting it deserves a separate discussion.