Browse Mode: Use heading elements for the "visual headings"

[carolinan]

Description

In the navigation sidebar in the browse mode there are a few texts that look like headings: They are using a larger text size and are positioned above and in close proximity to other content; the words design, templates and template parts.

These need to be actual heading elements to allow navigating via headings.
For a more detailed explanation of why this is needed please see #42373 and #42495 which were opened before the introduction of the browse mode.

This is in addition to the missing H1 that is already reported.

Step-by-step reproduction instructions

In the WordPress admin, select Appearance > Editor.
Once in the Site Editor, confirm that there are no headings on the page.

Screenshots, screen recording, code snippet

No response

Environment info

No response

Please confirm that you have searched existing issues in the repo.

Yes

Please confirm that you have tested with all plugins deactivated except Gutenberg.

Yes