'User guide' docs: how to use ApplicationSet controller (examples of generators, template fields)

[jgwest]

Can either be addressed standalone, or as a follow up to the parent issue

Showcase examples of each of the generators, and an explanations of how those examples work.

To include in the docs:

The design proposal is well written, and the content could likely be used for documentation of:

• How to use the ApplicationSet controllers:
  • specific examples of the following generators:
    • list generator
    • cluster generator
    • git generator
      • files
      • directories