adding turtle examples (#598)

docs/examples/turtle-scanner.md

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+# Turtle Scanner
+
+The turtle keeps on the scanning the screen.
+
+```blocks
+turtle.setPosition(0, 0)
+turtle.turnRight()
+turtle.setSpeed(20)
+basic.forever(() => {
+    turtle.forward(4)
+    turtle.turnRight()
+    turtle.forward(1)
+    turtle.turnRight()
+    turtle.forward(4)
+    turtle.turnLeft()
+    turtle.forward(1)
+    turtle.turnLeft()
+})
+```
+
+```package
+microturtle=github:Microsoft/pxt-microturtle#master
+```

docs/examples/turtle-spiral.md

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+# Turtle Spiral
+
+The turtle goes to the center back and forth.
+
+```blocks
+let index = 0
+turtle.setPosition(0, 0)
+turtle.turnRight()
+basic.forever(() => {
+    for (let index = 0; index <= 4; index++) {
+        turtle.forward(4 - index)
+        turtle.turnRight()
+    }
+    for (let index = 0; index <= 4; index++) {
+        turtle.turnLeft()
+        turtle.back(index)
+    }
+})
+```
+
+```package
+microturtle=github:Microsoft/pxt-microturtle#master
+```

docs/examples/turtle-square.md

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+# Turtle Square
+
+A turtle program that runs in a square.
+
+```blocks
+turtle.setPosition(0, 0)
+turtle.turnRight()
+turtle.setSpeed(29)
+basic.forever(() => {
+    turtle.forward(4)
+    turtle.turnRight()
+})
+```
+
+```package
+microturtle=github:Microsoft/pxt-microturtle#master
+```