[Documentation]: Customising top node

[plushdohn]

What’s the URL to the page you’re sending feedback for?

https://tiptap.dev/docs/editor/api/nodes/document

What part of the documentation needs improvement?

The Document extension docs are the only ones that mention a top node, but they don't describe how it can extended or customised (if possible at all)

What is helpful about that part?

The Document extension docs do explain what a top node is and how simple the extension's implementation is.

What is hard to understand, missing or misleading?

The extension's docs don't explain how it can be extended or if there are limitations in doing so.

Anything to add? (optional)

If a use-case is needed, in my editor I'm trying to let users customise the document's background and page margin.