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 don't think it's necessary to list all the reasons why it shouldn't be used - we aren't recommending against it (in which case details would be useful for user to make informed decision), we are outright telling them not to.

Perhaps just add the word "security"  somewhere in there.