[Docs] Add doc and quick demo for gpt-oss responses api & buildin tools (#9497)

Signed-off-by: Xinyuan Tong <xinyuantong.cs@gmail.com>

docs/basic_usage/gpt_oss.md

 # GPT OSS Usage
 
 Please refer to [https://github.com/sgl-project/sglang/issues/8833](https://github.com/sgl-project/sglang/issues/8833).
+
+## Responses API & Built-in Tools
+
+### Responses API
+
+GPT‑OSS is compatible with the OpenAI Responses API. Use `client.responses.create(...)` with `model`, `instructions`, `input`, and optional `tools` to enable built‑in tool use.
+
+### Built-in Tools
+
+GPT‑OSS can call built‑in tools for web search and Python execution. You can use the demo tool server or connect to external MCP tool servers.
+
+#### Python Tool
+
+- Executes short Python snippets for calculations, parsing, and quick scripts.
+- By default runs in a Docker-based sandbox. To run on the host, set `PYTHON_EXECUTION_BACKEND=UV` (this executes model-generated code locally; use with care).
+- Ensure Docker is available if you are not using the UV backend. It is recommended to run `docker pull python:3.11` in advance.
+
+#### Web Search Tool
+
+- Uses the Exa backend for web search.
+- Requires an Exa API key; set `EXA_API_KEY` in your environment. Create a key at `https://exa.ai`.
+
+## Notes
+
+- Use **Python 3.12** for the demo tools. And install the required `gpt-oss` packages.
+- The default demo integrates the web search tool (Exa backend) and a demo Python interpreter via Docker.
+- For search, set `EXA_API_KEY`. For Python execution, either have Docker available or set `PYTHON_EXECUTION_BACKEND=UV`.
+
+Examples:
+```bash
+export EXA_API_KEY=YOUR_EXA_KEY
+# Optional: run Python tool locally instead of Docker (use with care)
+export PYTHON_EXECUTION_BACKEND=UV
+```
+
+Launch the server with the demo tool server:
+
+`python3 -m sglang.launch_server --model-path openai/gpt-oss-120b --tool-server demo --tp 2`
+
+For production usage, sglang can act as an MCP client for multiple services. An [example tool server](https://github.com/openai/gpt-oss/tree/main/gpt-oss-mcp-server) is provided. Start the servers and point sglang to them:
+```bash
+mcp run -t sse browser_server.py:mcp
+mcp run -t sse python_server.py:mcp
+
+python -m sglang.launch_server ... --tool-server ip-1:port-1,ip-2:port-2
+```
+The URLs should be MCP SSE servers that expose server information and well-documented tools. These tools are added to the system prompt so the model can use them.
+
+### Quick Demo
+
+```python
+from openai import OpenAI
+
+client = OpenAI(
+    base_url="http://localhost:30323/v1",
+    api_key="sk-123456"
+)
+
+tools = [
+    {"type": "code_interpreter"},
+    {"type": "web_search_preview"},
+]
+
+# Test python tool
+response = client.responses.create(
+    model="openai/gpt-oss-120b",
+    instructions="You are a helfpul assistant, you could use python tool to execute code.",
+    input="Use python tool to calculate the sum of 29138749187 and 29138749187", # 58,277,498,374
+    tools=tools
+)
+print("====== test python tool ======")
+print(response.output_text)
+
+# Test browser tool
+response = client.responses.create(
+    model="openai/gpt-oss-120b",
+    instructions="You are a helfpul assistant, you could use browser to search the web",
+    input="Search the web for the latest news about Nvidia stock price",
+    tools=tools
+)
+print("====== test browser tool ======")
+print(response.output_text)
+```
+
+Example output:
+```
+====== test python tool ======
+The sum of 29,138,749,187 and 29,138,749,187 is **58,277,498,374**.
+====== test browser tool ======
+**Recent headlines on Nvidia (NVDA) stock**
+
+| Date (2025) | Source | Key news points | Stock‑price detail |
+|-------------|--------|----------------|--------------------|
+| **May 13** | Reuters | The market data page shows Nvidia trading “higher” at **$116.61** with no change from the previous close. | **$116.61** – latest trade (delayed ≈ 15 min)【14†L34-L38】 |
+| **Aug 18** | CNBC | Morgan Stanley kept an **overweight** rating and lifted its price target to **$206** (up from $200), implying a 14 % upside from the Friday close. The firm notes Nvidia shares have already **jumped 34 % this year**. | No exact price quoted, but the article signals strong upside expectations【9†L27-L31】 |
+| **Aug 20** | The Motley Fool | Nvidia is set to release its Q2 earnings on Aug 27. The article lists the **current price of $175.36**, down 0.16 % on the day (as of 3:58 p.m. ET). | **$175.36** – current price on Aug 20【10†L12-L15】【10†L53-L57】 |
+
+**What the news tells us**
+
+* Nvidia’s share price has risen sharply this year – up roughly a third according to Morgan Stanley – and analysts are still raising targets (now $206).
+* The most recent market quote (Reuters, May 13) was **$116.61**, but the stock has surged since then, reaching **$175.36** by mid‑August.
+* Upcoming earnings on **Aug 27** are a focal point; both the Motley Fool and Morgan Stanley expect the results could keep the rally going.
+
+**Bottom line:** Nvidia’s stock is on a strong upward trajectory in 2025, with price targets climbing toward $200‑$210 and the market price already near $175 as of late August.
+
+```