Rewrite deployment docs

- [x] Many users falsely assume Flask's builtin server is suitable for
  deployment. Unfortunately the quickstart didn't explicitly mention this at
  all.

- [x] The change from ``app.run`` to the ``flask`` command has left some traces.

- [x] Not even the deployment docs themselves mention how bad the builtin
  server is, instead they plainly say "don't use it".
Meh, adding "security" as some sort of generic scarecrow seems bad. If we're going to say anything about ``werkzeug.serving``'s security, we should mention that it's still better than Py2's ``wsgiref``.