[*-apps] We need usage docs for argoconfig and *-apps charts

[hairmare]

Affected chart
All (potential) *-apps charts

Describe the bug
The auto-generated documentation for the charts is currently lacking. The examples/ dirs in the charts have proven to not contain enough details for end users.

Expected behavior
There should be nice end-user quality docs for *-apps charts and the used corresponding library charts.

Additional context
The *-apps charts aim at being as generic as possible so documenting them in the top-level docs/ probably makes most sense.

Tasks

• initialize *-apps docs docs: initialise Argo CD app of apps documentation #427
• explain how to use argoconfig docs(app-of-apps): argoconfigs template #477
• explain app-of-apps chart structure docs(app-of-apps): add info on app-of-apps chart structure #499
• explain app-of-apps concept docs: initialise Argo CD app of apps documentation #427
• highlight some example charts (infra-apps, security-apps, and maybe a third one), show how to find them on artifacthub
• explain initial deployment & boostrap (esp. regarding infra-apps and the (env-)main-argo app and argocd-configs repo)
• add guidelines that explain when to add stuff to a *-apps (and, more important, when not to)
• document how to use $app.name to override $app.destination.namespace based app name
• explain how to test changes to by rolling them out to a test-cluster

[eyenx]

@tongpu suggested documenting values like syncPolicy inside the -apps/values.yaml but I'm against this, as this values come from the argoconfig library chart.

How do we want to make sure people know, they can set these values? Extend argoconfig's documentation is enough?

[hairmare]

At some we might want to add some high-level docs to the docs/ folder. Those might describe how to use the app-of-apps things based on argoconfig.