Add custom gallery thumbnails for animated and fixed examples

- Added animated GIF thumbnails for 6 examples: chemtrails, animate_MDS, animate_spin, animate, precog, save_movie
- Added custom static PNG thumbnails for 3 examples: explore, save_image, analyze
- All GIFs maintain 50fps framerate with infinite looping enabled
- Thumbnails stored in docs/custom_thumbnails/ for version control

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <[email protected]>

Author: jeremymanning