Add Colab quickstarts for three flagship loops

[rsasaki0109]

Goal

Let readers run the strongest examples without setting up a local checkout first.

Notebooks

• pick_and_retry: grasp miss -> belief update -> retry.
• safety_filter_cbf: nominal controller -> CBF projection -> safe motion.
• active_slam_toy: map and pose uncertainty -> information-seeking action.

Tasks

• Add a small notebooks/ directory or documented external Colab links.
• Keep notebooks minimal: install, import, run headless, display GIF or generated frames.
• Add README links after the notebooks are tested.

Acceptance Criteria

• Each notebook runs from a fresh Colab runtime.
• The notebooks do not require ROS, GPU, Docker, or a heavy simulator.
• README has a compact Colab section or badges.