[FR] Outline for subpages

### Description

Outline for page can be created using `/outline`. It uses headlines.
The same functionality should be for creating the outline for sub pages. It would allow easy navigation in nested documents structure.

It could look like this:

### Impact

Easy navigation through sub documents.
I like to have a nested structure of smaller documents. This would enable to have the top document as a overview of the structure. One could easily navigate to the specific document.

### Additional Context

I don't have a good idea for the keyword when using '/'.
Maybe 'structure', 'overview'? Any suggestions?