PIPE-2738: Improve image visibility with drop shadows and click-to-zoom lightboxes
- Added a subtle drop shadow and border in `custom_theme.css` to help embedded screenshots and figures stand out clearly against the page background.
- Enabled the `sphinxcontrib-lightbox2` Sphinx extension (via `conf.py` and `requirements_docs.txt`) to allow users to click and expand images into full-screen overlays.
- Updated `pixi.lock` to lock the new Sphinx extension and refresh environment dependencies.
