Documentation for built-in Zarf variables (i.e. ZARF_REGISTRY)

[runyontr]

As Ezra I want to see what built-in templates are available so that I can use them in my own packages.

Acceptance Criteria

Given I am on docs.zarf.dev
When I visit a new page under https://docs.zarf.dev/docs/create-a-zarf-package/
Then I will see a new page discussing package defined variables and built-in deploy-time variables
And When I select that page
Then I will see sections for Variables, Constants and all of the built in templates that Zarf has.

https://github.com/defenseunicorns/zarf/blob/main/src/internal/packager/template/template.go#L75

Additional Context

I was trying to identify what zarf variables are automatically available to zarf package creators, e.g. something like ###ZARF_PKG_NAME#### or ###ZARF_PKG_NAMESPACE### but couldn't find any documentation on it.

[Racer159]

@Madeline-UX here is another example of Docs <> Examples <> Walkthroughs information architecture mixups.

@runyontr there are some docs here: https://docs.zarf.dev/examples/package-variables/#how-to-use-create-time-package-variables

[Noxsios]

My initial thoughts would be an addition to make docs-and-schema to add a go run main.go internal gen-variables-docs (or something along those lines)