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".
How about:

"Flask's built-in server is lightweight and easy to use however it is **not suitable for production use** as it doesn't scale well and by default will only serve one request at a time.