docs(robotwin): add robotwin to _toctree.yml under Benchmarks

doc-builder's TOC integrity check was rejecting the branch because
docs/source/robotwin.mdx existed but wasn't listed in _toctree.yml.

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: robotwin
+    title: RoboTwin 2.0
   - local: envhub_isaaclab_arena
     title: NVIDIA IsaacLab Arena Environments
   title: "Benchmarks"