Rebuild the docs around tested examples; shrink README.v2.md to a pitch (#2978)

docs_src/structured_output/tutorial004.py

@@ -0,0 +1,18 @@
+from dataclasses import dataclass
+
+from mcp.server import MCPServer
+
+mcp = MCPServer("Weather")
+
+
+@dataclass
+class WeatherData:
+    temperature: float
+    humidity: float
+    conditions: str
+
+
+@mcp.tool()
+def get_weather(city: str) -> WeatherData:
+    """Current weather for a city."""
+    return WeatherData(temperature=16.2, humidity=0.83, conditions="Overcast")