docs: restructure into topical sections and add the four most-asked-for pages (#3044)

docs_src/troubleshooting/tutorial003.py

@@ -0,0 +1,12 @@
+from mcp.server import MCPServer
+
+mcp = MCPServer("Weather")
+
+
+@mcp.tool()
+def forecast(city: str) -> str:
+    """Today's forecast for one city."""
+    return f"{city}: Rain."
+
+
+app = mcp.streamable_http_app()