docs: add silent cel shading shader to URP list

Author: towneh

content/docs/avatar/shaders.mdx

@@ -18,6 +18,12 @@ import { Callout } from 'fumadocs-ui/components/callout';
 
 ![Poiyomi Shader](https://s2.booth.pm/430bb41d-8aaa-46f4-ae48-63cff7c3ff3e/132c3ac8-cc2f-4858-9a92-2844c7f5cc05.png)
 
+### Silent Cel Shading Shader (MIT License) by s-ilent
+
+- https://github.com/s-ilent/scss or https://gitlab.com/s-ilent/SCSS
+
+![Silent Cel Shading Shader](https://booth.pximg.net/da52898a-55fb-47ad-bd28-d1e52668811d/i/1894974/404925a9-044e-4a8e-8ea6-fb395ba45dab.png)
+
 ### lilToon (MIT License) by lilxzw
 
 - https://github.com/lilxyzw/lilToon