DOC: Add gallery example for using EPSG codes

examples/gallery/maps/epsg_codes.py

@@ -0,0 +1,23 @@
+"""
+EPSG codes
+==========
+
+Besides one of the :doc:`31 projections supported by GMT </techref/projections>`, users
+can pass an EPSG (European Petroleum Survey Group) code to the ``projection`` parameter
+of the methods :meth:`Figure.basemap` and :meth:`Figure.coast`. A commonly used EPSG
+code is ``EPSG:3857``, that refers to the Web Mercator projection WGS84. More
+information on the EPSG dataset can be found at https://epsg.org/home.html.
+
+"""
+
+# %%
+import pygmt
+
+fig = pygmt.Figure()
+
+# Pass the desired EPSG code and the width of the map, here 10 centimeters, to the
+# projection parameter
+fig.basemap(region=[-180, 180, -60, 60], projection="EPSG:3857/10c", frame="30")
+fig.coast(shorelines="1/0.1p,gray10")
+
+fig.show()