Document or remove default superuser account

[fluix-dev]

The demo fixture creates a default account with a username and password of admin. This superuser account allows completely access to everything on the site, and isn't documented anywhere. Finding DMOJ instances with this account left is common in the Slack help channels.

Because the documentation explains how to create a superuser account shortly after the loaddata command, the default admin:admin one should be removed.

[Xyene]

The docs should be fixed, we can't just remove the admin account from the fixture since most of the demo data references it.