Provide an example destination URL

When I tried to work out how to configure a sbws destination, I looked at:

• DEPLOY.rst, but the links were broken (#28648 (moved))
• config.default.ini, but the [destinations] section didn't tell me what I needed to add
• man_sbws.ini.rst , but I couldn't work out what needs to go in the minimal sbws.ini
• an example config.ini, but there isn't one

Please:

• add an example URL to config.default.ini, or
• add a config.ini with an example URL

Please also:

• clearly document the minimal sbws.ini in DEPLOY.rst, and
• clearly document the minimal sbws.ini at the top of man_sbws.ini.rst
• tell me I shouldn't change the other options listed in man_sbws.ini.rst

This ticket is required for sbws setup, so it should go in sbws 1.0.