docs/production: Reorganize SMTP docs a bit.
docs/production: Reorganize SMTP docs a bit.

Let's get right to the point of how to configure SMTP once you know
what you want.  That section is pretty short anyway; and we can have
a first step direct the reader to our suggestions if they don't know
what service they want to use.

Also adjust the hierarchy of the headings: group the various
alternative email services under one heading, and group
troubleshooting together under an independent heading.

Also correct what we say about EMAIL_PORT: the Django default is
apparently 25, so if the provider *does* use the usual port 587
then we'll need the port to be set.

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