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

docs_src/run/tutorial003.py

@@ -0,0 +1,13 @@
+from mcp.server import MCPServer
+
+mcp = MCPServer("Bookshop", log_level="DEBUG")
+
+
+@mcp.tool()
+def search_books(query: str) -> str:
+    """Search the catalog by title or author."""
+    return f"Found 3 books matching {query!r}."
+
+
+if __name__ == "__main__":
+    mcp.run()