'User guide' docs: the whats/whys/hows of ApplicationSet controller

[jgwest]

Write new documentation content, similar to Argo CD's user guide, on how to use ApplicationSet controller.

To include in the docs:

The design proposal is well written, and that content could likely be used for new user-facing documentation of:

• What ApplicationSet is
  • still experimental, feedback welcome
• Why does it exist, what problem is ApplicationSet controller looking to solve, versus Argo CD proper
  • scenarios
    • cluster add-ons
    • monorepo
    • self-service
  • issues with the app-of-apps pattern for these use cases
• How ApplicationSet works
  • what are generators? (general introduction)
  • what are the template fields? (general introduction, eg what are src, dest for)
  • how it interacts with Argo CD proper
  • Note: Don't need to document any specific, detailed examples of the generators as part of this issue, they will be addressed separately as part of this issue

Docs to be written in the same style as Argo CD:

• Create a docs/ in ApplicationSet repo, docs to be written in markdown
• Docs to follow the defacto 'style guide' of the existing Argo CD user guide (eg same use of # and ## to denote sections and subsections, and following any other desired traits/idioms of those existing docs)
• This issue is for writing the content only, no need to setup 'rendering markdown to HTML' step as part of this.