Add Applets buttons beside notebook Colab badges

[dellaert]

Add an Applets button beside the existing Colab badge in all 33 notebooks with standalone applet pages. Each button opens that section’s applets on roboticsbook.org, including S24’s S24_perception.html destination.

The compact button uses a brass gradient, a thin border, and a small decorative gear. Its styling is included in the notebook’s existing badge cell. Update the applet source guide to describe these book-to-applet links.

Validation:

• Verified all 33 destinations against the committed applet files.
• Compared notebook JSON before and after: only each existing badge cell’s source changed; code, outputs, metadata, and cell indices are preserved.
• Built the book locally with Jupyter Book 1.0.4.post1 using saved notebook outputs, then checked both adjacent links and preserved styling on all 33 rendered pages. The build completed with existing content/reference warnings.
• Visually inspected S24 in the local browser preview: both badges align and the new styling renders correctly.
• git diff --check passed.

Local review: http://127.0.0.1:8767/S24_sorter_perception.html (served on the development machine).