Add KDoc documentation to Gradient.kt

Summary:
Added KDoc documentation to the Gradient interface:

- Interface documentation explaining CSS gradient to Android Shader conversion
- getShader() method documentation with parameter and return descriptions
- Added see references to implementations

changelog: [internal] internal

Reviewed By: javache

Differential Revision: D91554867

Author: mdvacca

packages/react-native/ReactAndroid/src/main/java/com/facebook/react/uimanager/style/Gradient.kt

@@ -9,6 +9,22 @@ package com.facebook.react.uimanager.style
 
 import android.graphics.Shader
 
+/**
+ * Interface representing a CSS gradient that can be rendered as an Android Shader.
+ *
+ * Implementations of this interface (such as [LinearGradient] and [RadialGradient]) convert CSS
+ * gradient definitions into Android Shader objects for rendering backgrounds.
+ *
+ * @see LinearGradient
+ * @see RadialGradient
+ */
 internal interface Gradient {
+  /**
+   * Creates an Android Shader for rendering this gradient.
+   *
+   * @param width The width of the area to fill
+   * @param height The height of the area to fill
+   * @return A Shader instance configured for this gradient
+   */
   fun getShader(width: Float, height: Float): Shader
 }