docs: #3613484 Show the Mode dropdown at the top of the documentation home page

Follow-up on #3613484.

The documentation home page opened with a sentence describing the Mode dropdown and no picture of it. This puts the toolbar screenshot directly under that sentence, so the first thing a reader sees is the control the page is describing.

Image: https://www.drupal.org/files/project-images/a2-2-mode-selector-toolbar.png

Verified with mkdocs build --strict in a python:3.12-slim container: the build passes and the image is present in the rendered index.html.

Depends on MR !7, which adds mkdocs.yml. This branch is cut from !7 (merged), so merge !7 (merged) first, or merge this one after it and the diff will reduce to the single documentation change.

Documentation only. No code, configuration or test changes.

Checkpoints

• File an issue
• Addition/Change/Update/Fix
• Reviewed by a human
• Code review by maintainers