Initializing the table of contents creates a transaction even in read only mode

[max-nextcloud]

Describe the bug

1. Document opens
2. Table of contents is initialized
3. Headings are parsed and get an id
4. ids are added to heading nodes creating a transaction
5. transaction updates document state leading to steps pushed even in read only mode.

To Reproduce

Steps to reproduce the behavior:

1. Create a markdown file with headings and a public read only share of it
2. observe network traffic
3. Notice step send in one of the push requests

Expected behavior
Opening the document read only should not alter the state.

Screenshots
will attach later

[max-nextcloud]

This is related to #5800 (comment)

[mejo-]

Probably the way forward would be to migrate extractHeadings() to use ProseMirror Decorations for setting the id attribute locally, instead of altering the document state. See this example.

[juliusknorr]

Ah sounds like a good idea, haven't thought of that with my comment in #5800 (comment)