Reasoning parser (#4000)

Co-authored-by: Lucas Pickup <lupickup@microsoft.com>

docs/index.rst

@@ -37,6 +37,7 @@ The core features include:
    backend/speculative_decoding.ipynb
    backend/structured_outputs.ipynb
    backend/function_calling.ipynb
+   backend/separate_reasoning.ipynb
    backend/custom_chat_template.md
    backend/quantization.md