[Doc] Add Parallel Draft Models (#35973)

Signed-off-by: <zihaoan2@amd.com>
Signed-off-by: zihaoanllm <zihaoan2@amd.com>
Co-authored-by: Harry Mellor <19981378+hmellor@users.noreply.github.com>

docs/features/speculative_decoding/parallel_draft_model.md

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+# Parallel Draft Models
+
+The following code configures vLLM to use speculative decoding where proposals are generated by [PARD](https://arxiv.org/pdf/2504.18583) (Parallel Draft Models).
+
+## PARD Offline Mode Example
+
+```python
+from vllm import LLM, SamplingParams
+
+prompts = ["The future of AI is"]
+sampling_params = SamplingParams(temperature=0.8, top_p=0.95)
+
+llm = LLM(
+    model="Qwen/Qwen3-8B",
+    tensor_parallel_size=1,
+    speculative_config={
+        "model": "amd/PARD-Qwen3-0.6B",
+        "num_speculative_tokens": 12,
+        "method": "draft_model",
+        "parallel_drafting": True,
+    },
+)
+outputs = llm.generate(prompts, sampling_params)
+
+for output in outputs:
+    prompt = output.prompt
+    generated_text = output.outputs[0].text
+    print(f"Prompt: {prompt!r}, Generated text: {generated_text!r}")
+```
+
+## PARD Online Mode Example
+
+```bash
+vllm serve Qwen/Qwen3-4B \
+    --host 0.0.0.0 \
+    --port 8000 \
+    --seed 42 \
+    -tp 1 \
+    --max_model_len 2048 \
+    --gpu_memory_utilization 0.8 \
+    --speculative_config '{"model": "amd/PARD-Qwen3-0.6B", "num_speculative_tokens": 12, "method": "draft_model", "parallel_drafting": true}'
+```
+
+## Pre-trained PARD weights
+
+- [amd/pard](https://huggingface.co/collections/amd/pard)