add docs

docs/lib2x-nz_perlin2.md

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+# nz_perlin2
+
+Returns the 2D [Perlin noise](https://en.wikipedia.org/wiki/Perlin_noise) value at the (x, y) coordinate.
+
+**Since:** 2.3
+
+## Parameters
+
+- `x` : The x coordinate.
+- `y` : The y coordinate.
+- `seed` : The random seed.
+
+## Examples
+
+    use <util/rand.scad>;
+    use <hull_polyline2d.scad>;
+    use <function_grapher.scad>;
+    use <noise/nz_perlin2.scad>;
+    use <contours.scad>;
+
+    seed = rand(0, 255);
+    points = [
+        for(y = [0:.1:10])
+        [
+            for(x = [0:.1:10])
+            [x, y, nz_perlin2(x, y, seed)]
+        ]
+    ];
+
+    function_grapher(points, 1);
+
+    translate([11, 0])
+    for(isoline = contours(points, 0)) {
+        hull_polyline2d(isoline, width = .1);
+    }  
+
+![nz_perlin2](images/lib2x-nz_perlin2-1.JPG)

docs/lib2x-nz_perlin2s.md

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+# nz_perlin2s
+
+Returns 2D [Perlin noise](https://en.wikipedia.org/wiki/Perlin_noise) values at (x, y) coordinates.
+
+**Since:** 2.3
+
+## Parameters
+
+- `points` : A list of `[x, y]` coordinates.
+- `seed` : The random seed. If it's ignored, a randomized value will be used.
+
+## Examples
+
+    use <util/rand.scad>;
+    use <hull_polyline2d.scad>;
+    use <function_grapher.scad>;
+    use <noise/nz_perlin2s.scad>;
+    use <contours.scad>;
+
+    seed = rand(0, 255);
+
+    for(y = [0:.1:10]) {
+        points = [for(x = [0:.1:10]) [x, y]];
+        noises = nz_perlin2s(points, seed);
+        for(i = [0:len(points) - 1]) {
+            translate(points[i])
+            linear_extrude(noises[i] + 1)
+                square(.1);
+        }
+    }
+
+![nz_perlin2s](images/lib2x-nz_perlin2s-1.JPG)