[edit] Enable document comments

A friction point with customers who want to reduce their content creation steps is that they need comments within a DA doc if they are going to consider moving away from Google Docs / Word.

## Criteria of acceptance
1. Ability to annotate pieces of content.
2. Works with ProseMirror / YJS
3. Does not get written out in a way that Helix will pick up.

## Additional considerations
1. A nice to have feature would be a new role for "commenter" / "collaborator" but this is extremely nice to have.

The issue names no files or tests; start by locating the document editor's ProseMirror/YJS integration. Define how content annotations are created and persisted without being written in a form Helix picks up, then verify the three acceptance criteria; the commenter role is optional.

Written by the indexing model from the issue text.