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".
OK, so I guess [this](https://github.com/mitsuhiko/flask/blob/flask_deployment_docs/docs/deploying/index.rst) GitHub view of it just doesn't show that?