Docs: Change settings supported to env variables supported?

[LEW21]

I think that most people using django12factor are using it in the "give me everything" mode (because otherwise they would use dj-database-url etc directly). This means, that they consider the Django settings set by the library as an implementation detail, and are mostly interested in which environment variables they can use. Also, users of these apps of course care only about the variables.

It might be a good idea to reorganize the docs, so that they focus on the "everything" mode, and document the variables from end-user perspective.

I can make a PR if you agree.