feat(ci): add benchmark smoke tests with isolated Docker images

Each benchmark gets its own image (lerobot[<benchmark>,smolvla]) so
incompatible dep trees can never collide. A 1-episode smoke eval runs
per benchmark on GPU runners.

- Libero: pepijn223/smolvla_libero, libero_spatial, camera_name_mapping
- MetaWorld: pepijn223/smolvla_metaworld, metaworld-push-v2
- LIBERO config pre-created at build time to bypass interactive stdin prompt
- Triggers on envs/**, lerobot_eval.py, Dockerfiles, pyproject.toml changes
- Adds docs/source/evaluation.mdx and restores step 7 in adding_benchmarks

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

docs/source/_toctree.yml

@@ -73,6 +73,8 @@
     title: Control & Train Robots in Sim (LeIsaac)
   title: "Simulation"
 - sections:
+  - local: evaluation
+    title: Evaluation (lerobot-eval)
   - local: adding_benchmarks
     title: Adding a New Benchmark
   - local: libero