docs: rename section for clarity in mainpage.md

- Changed the section title from "Quick Start" to "Simple Example" to better reflect the content and improve user understanding.

Signed-off-by: Y.Hisaki <yhisaki31@gmail.com>

Author: yhisaki

docs/mainpage.md

@@ -95,7 +95,7 @@ The library uses a **client-server architecture**:
 
 You can find more details in [Architecture Overview](architecture.md).
 
-## Quick Start
+## Simple Example
 
 ```cpp
 #include "plotly/plotly.hpp"