Docs: Wagtail settings variables and urlconfs

The goals for this docs page are (relating to https://github.com/torchbox/wagtail/issues/295):

* Document Wagtail-specific settings variables
* Provide enough info to bootstrap a Wagtail install without cloning the demo site

This PR also sprawls a bit into adding a small section on Wagtail Redirects, adding a TOC to the Django Devs section, and fixing a link in the Editing API doc.

Might be a good idea to set this to "Change me" so people don't end up using this key in their projects.