Add the introduction video to the home page and the README

[pip-install-python]

"Dash Leaflet 2.0: Drone Tracking, Image Overlays & Map Packages in Python" — https://youtu.be/Wlmw98JrJZI

docs/home/video.py is its own module rather than part of example.py, because the two are different things: example.py is the live Leaflet 2 demo that proves the library renders, and this is a recording that explains it. .. exec:: imports each by module path, so the markdown places them independently.

Three details worth keeping:

• embedded from youtube-nocookie.com, so the player sets none of the visitor-tracking cookies the standard embed does -- this site otherwise counts nothing about a reader beyond an anonymised page view, and the embed should not quietly undo that;
• fullscreen goes in the iframe's allow list, not in an allowFullScreen prop. Dash 4.4.1's html.Iframe does not expose the legacy attribute at all, and without the permission the player still renders a fullscreen button that silently does nothing;
• a plain markdown link accompanies the player. An agent reading /llms.txt never sees an iframe, and neither does anyone whose browser blocks the embed -- verified that /llms.txt names the URL and that a crawler receives the link in the prerendered body.