docs(robotwin): use the {video} MyST directive so the 50-task gallery actually plays

The raw <video src=/roboverse/_static/...> HTML (copied from old simpler_env) does
not play on the built site (same issue fixed for simpler_env in 99480d9). Switch
all 50 clips to the sphinxcontrib.video ```{video}``` directive + {grid}/{grid-item},
matching the working simpler_env page. Verified: sphinx build emits 50 <video> tags
with resolved _images/ sources.

Author: geng-haoran