Page Block
This is the root node of the document tree, and its view implementation becomes the top-level UI of the editor.
For instance, the doc editor and the edgeless editor implement two different views for the page block. Generally, content (leaf nodes) is not placed directly in the page block. Its direct children are usually at least one note block and one optional surface block. Rich text content like paragraphs and lists are generally placed in the note blocks, while graphical content is placed in the surface block.
