fix(docs): SimplerEnv gallery videos use the {video} directive (were not playing)

The raw <video src="/roboverse/_static/..."> tags used an absolute path that
sphinx does not rewrite, so the clips 404'd on local builds and any base other
than the deployed /roboverse/ — none played. Switch all 25 to sphinxcontrib.video
{video} directives (source-relative path; sphinx copies to _images/ and rewrites
the src per page), laid out in sphinx-design 2-col grids with per-task captions.
Verified by a full sphinx build: 25 <video> tags, src -> ../../_images/<task>.mp4
which exists in the build output, 0 warnings.

Author: geng-haoran