[MM][Doc] Update online serving tutorials for Qwen2-Audio (#3606)

### What this PR does / why we need it?
Update online serving tutorials for `Qwen2-Audio`.

Part of https://github.com/vllm-project/vllm-ascend/issues/3508.

### Does this PR introduce _any_ user-facing change?

### How was this patch tested?

- vLLM version: v0.11.0rc3
- vLLM main: https://github.com/vllm-project/vllm/commit/v0.11.0

---------

Signed-off-by: shen-shanshan <467638484@qq.com>

examples/chat_templates/template_qwen2_audio.jinja

@@ -0,0 +1,23 @@
+{% set audio_count = namespace(value=0) %}
+{% for message in messages %}
+    {% if loop.first and message['role'] != 'system' %}
+        <|im_start|>system\nYou are a helpful assistant.<|im_end|>\n
+    {% endif %}
+    <|im_start|>{{ message['role'] }}\n
+    {% if message['content'] is string %}
+        {{ message['content'] }}<|im_end|>\n
+    {% else %}
+        {% for content in message['content'] %}
+            {% if 'audio' in content or 'audio_url' in content or message['type'] == 'audio' or content['type'] == 'audio' %}
+                {% set audio_count.value = audio_count.value + 1 %}
+                Audio {{ audio_count.value }}: <|audio_bos|><|AUDIO|><|audio_eos|>\n
+            {% elif 'text' in content %}
+                {{ content['text'] }}
+            {% endif %}
+        {% endfor %}
+        <|im_end|>\n
+    {% endif %}
+{% endfor %}
+{% if add_generation_prompt %}
+    <|im_start|>assistant\n
+{% endif %}