Add documentation for arm update

implementation/SCARA/06_arm/implementation.md

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+## Complex Arm Design.
+
+So I improved the arm design.
+With my modern robotics library this became very very easy.
+
+Although, I forgot to set all the variables correctly.
+This resulted in a small arm piece to have a mass of 0.5 kg.
+
+Anyway, after I resolved that, the design worked very well.
+The controllers are reduced to a double proportional controller.
+The operation is really good.