docs: refresh README hero with an editor game-scene screenshot

Swap the hero image for a Celestial Heights action-RPG scene open in the
editor, with its scene graph populated in the outliner.

Author: esengine

README.md

@@ -22,7 +22,9 @@
 Estella is a **2D game engine** with a TypeScript SDK driven by a high-performance **C++/WebAssembly** backend. It ships with a visual editor for scene editing and project management, and outputs games that run in **web browsers** and **WeChat MiniGames**.
 
 <div align="center">
-  <img src="docs/assets/editor-screenshot.png" alt="Estella Editor" width="800" />
+  <img src="docs/assets/editor-screenshot.png" alt="An action-RPG scene open in the Estella editor" width="900" />
+  <br/>
+  <sub><i>Celestial Heights — an action-RPG scene in the Estella editor, with the full scene graph in the outliner.</i></sub>
 </div>
 
 ## Why Estella?