DOC: Add gallery example for using EPSG codes (#3973)

Co-authored-by: Dongdong Tian <[email protected]>

Author: yvonnefroehlich

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.
+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 and
+https://spatialreference.org/. Please note, that not all EPSG codes are supported by
+GMT / PyGMT.
+"""
+
+# %%
+import pygmt
+
+fig = pygmt.Figure()
+
+# Pass the desired EPSG code and the width of the map (separated by a slash) to the
+# projection parameter
+fig.basemap(region=[-180, 180, -60, 60], projection="EPSG:3857/10c", frame=30)
+fig.coast(land="gray", shorelines="1/0.1p,gray10")
+
+fig.show()