Docs: Refer to QUnit.module() callback as "scope" instead of "nested"

* Reserve use of the word "nested" for when multiple modules are nested
  as child in a parent. In general, refer to it simply as a scope.

* Move "Module scope" up as first section, followed by Hooks,
  and lastly Options.

* Embed basic example straight into the first section instead of buried
  far down.

* Simplify and shorten several examples to draw attention to the
  structure and away from the example code.