Draft: Resolve "Document Outline in Source Editor markdown code view"

What does this MR do and why?

Introduces the new action for Source Editor with a keystroke (cmd + J (as in jump) as per #348928) to show the table of contents when editing a markdown file in the Source Editor.

Screenshots or screen recordings

The video of the current draft

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

Related to #348783