Move placo examples and test in a specific folder

src/example/placo/README.md

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+# Usage
+
+For these examples, you will need [Placo](https://github.com/pollen-robotics/reachy_placo).

src/example/show_graph_singularity.py

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+import matplotlib.pyplot as plt
+import numpy as np
+
+
+def make_graph() -> None:
+    """make a 3D graph of the shoulder pitch joint depending on the x and y position of the elbow on the shoulder frame"""
+    x = np.linspace(-0.1, 0.1, 500)
+    y = np.linspace(-0.1, 0.1, 500)
+    x, y = np.meshgrid(x, y)
+    z = -np.arctan2(y, x) % (2 * np.pi)
+    fig = plt.figure()
+    ax = fig.add_subplot(111, projection="3d")
+    ax.plot_surface(x, y, z, cmap="viridis")
+    ax.set_xlabel("X")
+    ax.set_ylabel("Y")
+    ax.set_zlabel("-atan2(y, x)")
+    plt.show()
+
+
+def main_test() -> None:
+    make_graph()
+
+
+if __name__ == "__main__":
+    main_test()