add docs

docs/lib3x-find_index.md

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+# find_index
+
+Returns the index of the first element in the list that satisfies the testing function. If no element passed the test, it returns -1.
+
+**Since:** 3.0
+
+## Parameters
+
+- `lt` : The list.
+- `test` : the testing function.
+
+## Examples
+
+    use <util/find_index.scad>;
+    
+    assert(find_index([10, 20, 30, 40], function(e) e > 10) == 1);

docs/lib3x-shuffle.md

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+# shuffle
+
+Randomizes the order of the elements of a list.
+
+**Since:** 3.0
+
+## Parameters
+
+- `lt` : The list to shuffle.
+- `seed` : Random seed value.
+
+## Examples
+
+    use <util/shuffle.scad>;
+    
+    echo(shuffle([1, 2, 3, 4]));

docs/lib3x-swap.md

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+# swap
+
+Swaps two elements in a list.
+
+**Since:** 3.0
+
+## Parameters
+
+- `lt` : The list.
+- `i` : The index of an element.
+- `j` : The index of the other element
+
+## Examples
+
+    use <util/swap.scad>;
+    
+    assert(swap([10, 20, 30, 40], 1, 3) == [10, 40, 30, 20]);