docs(integrations): add SimplerEnv 1:1 integration page

New docs/source/dataset_benchmark/integrations/simpler_env.md (wired into the
index toctree): architecture of the two-track integration (MetaSim-native +
optional upstream passthrough), environment setup, usage, reproduce commands,
the parity table (22/25 bitwise Δ=0, 3 WidowX sub-pixel, success 25/25), design
notes and honest caveats.

Includes a per-task 1:1 gallery: for all 25 tasks a side-by-side
[ MetaSim-native | reference (=upstream) | abs-diff x30 ] clip (the diff panel is
near-black), grouped by the 6 task families. Videos under
_static/integrations/simpler_env/ (each <500KB), embedded with the repo's proven
<video autoplay loop muted playsinline> + absolute /roboverse/_static/ pattern.

Author: geng-haoran