docs: add JavaDoc to AreaOfPolygon algorithm

Author: dyrpsf

src/main/java/com/thealgorithms/maths/AreaOfPolygon.java

@@ -0,0 +1,40 @@
+/**
+ * This class provides a method to calculate the area of a simple polygon
+ * using the Shoelace formula (Gauss's area formula).
+ *
+ * The vertices should be ordered either clockwise or counter-clockwise.
+ */
+public class AreaOfPolygon {
+
+    /**
+     * Calculates the area of a polygon using x and y coordinates.
+     * Uses the Shoelace formula:
+     * Area = (1/2) * |x[i]*y[i+1] - x[i+1]*y[i]|
+     *
+     * @param x x-coordinates of the polygon's vertices
+     * @param y y-coordinates of the polygon's vertices
+     * @return area of the polygon
+     * @throws IllegalArgumentException if x and y are different lengths
+     */
+    public static double area(double[] x, double[] y) {
+        if (x.length != y.length) {
+            throw new IllegalArgumentException("Arrays must be same length");
+        }
+
+        double sum = 0.0;
+        int n = x.length;
+
+        for (int i = 0; i < n; i++) {
+            sum += (x[i] * y[(i + 1) % n]) - (x[(i + 1) % n] * y[i]);
+        }
+
+        return Math.abs(sum) / 2.0;
+    }
+
+    // Sample usage
+    public static void main(String[] args) {
+        double[] x = {0, 4, 4, 0};
+        double[] y = {0, 0, 4, 4};
+        System.out.println("Area: " + area(x, y)); // Should print 16.0
+    }
+}