docs: autogen built-in variables documentation

[Noxsios]

Describe what should be investigated or refactored

There are many built-in variables that are under-documented, or contain no documentation outside of usages in examples or core packages. Similar to the CLI documentation, there should be an autogenerated page/table in our docs to showcase each variable, its use and its default.

Links to any relevant code

#2330 (comment)

Additional context

Add any other context or screenshots about the technical debt here.