docs(robocerebra): add benchmark page and toctree entry

Add a dedicated docs page for RoboCerebra that points at the canonical
dataset lerobot/robocerebra_unified and shows how to run eval + fine-tune
against it. Wire it into the Benchmarks section of the toctree so
doc-builder picks it up.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

docs/source/_toctree.yml

@@ -79,6 +79,8 @@
     title: LIBERO
   - local: metaworld
     title: Meta-World
+  - local: robocerebra
+    title: RoboCerebra
   - local: envhub_isaaclab_arena
     title: NVIDIA IsaacLab Arena Environments
   title: "Benchmarks"