add dedup doc

docs/lib2x-dedup.md

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+# dedup
+
+Eliminating duplicate copies of repeating vectors. If `lt` has many elements, sorting `lt` first and setting `sorted` to `true` will be faster.
+
+**Since:** 2.3
+
+## Parameters
+
+- `lt` : A list of vectors.
+- `sorted` : If `false` (default), use native `search`. If `true`, `lt` must be sorted by zyx (from the last idx to first) and `dedup` will use binary search internally.
+
+## Examples
+
+    use <pixel/px_circle.scad>;
+    use <util/dedup.scad>;
+
+    pts1 = px_circle(10, filled = true);
+    pts2 = [for(p = px_circle(5, filled = true)) p + [10, 0]];
+
+    // simple union
+    pts3 = dedup(concat(pts1, pts2));
+    for(p = pts3) {
+        translate(p)
+            square(1, center = true);
+    }
+
+![dedup](images/lib2x-dedup-1.JPG)
+
+    use <pixel/px_circle.scad>;
+    use <util/sort.scad>;
+    use <util/dedup.scad>;
+
+    pts1 = px_circle(20, filled = true);
+    pts2 = [for(p = px_circle(10, filled = true)) p + [20, 0]];
+
+    sorted_pts = sort(sort(concat(pts1, pts2), by = "x"), by = "y");
+    
+    // simple union
+    pts3 = dedup(sorted_pts, sorted = true);
+    for(p = pts3) {
+        translate(p)
+            square(1, center = true);
+    }
+
+![dedup](images/lib2x-dedup-2.JPG)