[FR] Commenting functionality and table of contents

### Description

Hey guys! Thanks for the great project.
I did not yet dug into the code here but rather ran into issues with flutter quill and am not sure how easy it is to fix them / found this project then.

So here is my two use cases and I would love to know, how easy it is to extend Appflowy editor with those or if there is already work being started on them:

1) **Adding support for comments**. What I would love to do is to add comments similar to how word / notion does it. So an inline comment symbol and then on the right side either in the same widget or in another widget the text of the comment.
2) Generating a **table of contents**. Similar to what notion does, just a simple overview of the heading which links on click to the relevant sections.

Maybe those are already on the roadmap? Keep up the good work and thanks a lot,
Matthias

### Impact

1) Pretty much any collaboration on documents would benefit a lot from commenting
2) Gain a quicker overview over complicated documents, this should be great for all users as well.

### Additional Context

_No response_