docs: add Excalidraw to the list of React real apps

[abdullahalmsaodi]

Summary

Add Excalidraw to the top of the Real Apps section as an example of a large, open-source React application.

Why Excalidraw?

Excalidraw is an open-source virtual whiteboard that provides a hand-drawn, sketch-like drawing experience directly in the browser.

I placed it at the top of the list because, at the time of this contribution, Excalidraw has the highest number of GitHub stars among the projects in this section, making it a particularly notable and widely recognized open-source React project.

It also provides a valuable real-world reference for developers looking to study a production-scale React application with complex interactions and UI behavior.

What It Demonstrates

• Interactive drawing and canvas-based editing
• Real-time collaboration
• Custom shapes and diagrams
• Exporting and sharing drawings
• Responsive user interfaces
• Complex keyboard and pointer interactions

Change

Added Excalidraw at the top of the Real Apps section:

- [excalidraw](https://github.com/excalidraw/excalidraw) - An open source virtual hand-drawn style whiteboard.

[abdullahalmsaodi]

Hi @enaqx ,
Could pls review and merge it?