[SuperEditor][SuperReader] - Facilitate documents within documents

[matthew-carroll]

Some common document layout features require what essentially amounts to a document within a document.

For example, every cell within a table is essentially its own document. A single cell can contain paragraphs, list items, images, etc.

Also, there's the concept of a banner, which is essentially a colored rounded rectangle that encompasses arbitrary content. That arbitrary content inside the banner is essentially a document within a document.

We'd like to solve this use-case in general so that any number of nodes, components, and layouts can be built with arbitrary content within a node. For this ticket, make it possible to display a document within a document within a document, which should stress test layout and content selection, without delving into unique problems such as rows and columns in tables.