Add led-matrix-painter example

Author: dsammaruga

examples/led-matrix-painter/README.md

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+# LED Matrix Designer
+
+This small example demonstrates a web-based 8x13 matrix designer. It provides:
+- 

examples/led-matrix-painter/app.yaml

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+name: Led Matrix Painter
+icon: 🟦
+description:
+  This example shows how to create a tool to design frames for an LED matrix using Arduino.
+  It provides a web interface where users can design frames and animations and export them as C/C++ code.
+
+bricks:
+  - arduino:web_ui
+  - arduino:dbstorage_sqlstore