Cross-ref the two-scales and secondary-axes examples. (matplotlib#30366)

An end user can easily first find e.g. the twin axes example while
looking for something like secondary axes, or vice versa.  Make it
easier for them to find whatever they really need.

Author: anntzer

galleries/examples/subplots_axes_and_figures/secondary_axis.py

@@ -9,6 +9,9 @@
 `.axes.Axes.secondary_yaxis`.  This secondary axis can have a different scale
 than the main axis by providing both a forward and an inverse conversion
 function in a tuple to the *functions* keyword argument:
+
+See also :doc:`/gallery/subplots_axes_and_figures/two_scales` for the case
+where two scales are not related to one another, but independent.
 """
 
 import datetime

galleries/examples/subplots_axes_and_figures/two_scales.py

@@ -12,7 +12,12 @@
 Such Axes are generated by calling the `.Axes.twinx` method. Likewise,
 `.Axes.twiny` is available to generate Axes that share a *y* axis but
 have different top and bottom scales.
+
+See also :doc:`/gallery/subplots_axes_and_figures/secondary_axis` for the case
+where the two scales are not independent, but related (e.g., the same quantity
+in two different units).
 """
+
 import matplotlib.pyplot as plt
 import numpy as np