add attention backend supporting matrix in the doc (#5211)

Co-authored-by: Stefan He <hebiaobuaa@gmail.com>

docs/index.rst

@@ -32,6 +32,7 @@ The core features include:
    backend/sampling_params.md
    backend/hyperparameter_tuning.md
    backend/structured_outputs_for_reasoning_models.ipynb
+   backend/attention_backend.md
 
 .. toctree::
    :maxdepth: 1