docs: add evaluation guide and update benchmarks doc

- New docs/source/evaluation.mdx covering lerobot-eval usage, batch_size
  auto-tuning, AsyncVectorEnv performance, tuning tips, output format,
  multi-task evaluation, and programmatic usage.
- Add evaluation page to _toctree.yml under Benchmarks section.
- Update adding_benchmarks.mdx to reference batch_size auto default and
  link to the evaluation guide.

Made-with: Cursor

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