Add non-geographic plots to projection gallery (#728)

* Add cartesian.py
* Add polar.py

examples/projections/nongeo/cartesian.py

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+"""
+Cartesian
+=========
+
+``Xwidth/[height]``: Give the ``width`` of the figure and the optional argument ``height``.
+"""
+import pygmt
+
+fig = pygmt.Figure()
+fig.plot(
+    # The ``x`` and ``y`` parameters are used to plot lines on the figure.
+    x=[3, 9, 2],
+    y=[4, 9, 37],
+    pen="3p,red",
+    # ``region`` sets the x and y ranges or the Cartesian figure.
+    region=[0, 10, 0, 50],
+    # The argument ``WSne`` is passed to ``frame`` to put axis labels only on the left and bottom axes.
+    projection="X15c/10c",
+    frame=["af", "WSne"],
+)
+fig.show()

examples/projections/nongeo/polar.py

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+"""
+Polar
+=====
+
+``Pwidth``: Give the ``width`` of the figure.
+
+"""
+import pygmt
+
+fig = pygmt.Figure()
+fig.plot(
+    # x inputs are the theta values for a polar plot.
+    x=[180, 120, 270, 60, 0],
+    # y inputs are the radius values for a polar plot.
+    y=[15, 35, 15, 35, 15],
+    pen="2p,blue",
+    # The region values are theta-min/theta-max/radius-min/radius-max.
+    region=[0, 360, 0, 40],
+    projection="P15c",
+    frame=["afg"],
+)
+fig.show()