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".
I'm unsure whether we should extend this to say that DoS attacks are therefore easily possible (see https://github.com/mitsuhiko/flask/issues/1266). It might scare off some more people, but "serves only one request at a time" already sounds more "frightening" to me.