Update matplotlib's canonical (#4919)

Please see: https://discourse.matplotlib.org/t/canonical-documentation-have-moved/21863

Author: jklymak

doc/conf.py

@@ -415,7 +415,7 @@
     "numpy": ("https://numpy.org/doc/stable", None),
     "scipy": ("https://docs.scipy.org/doc/scipy/reference", None),
     "numba": ("https://numba.pydata.org/numba-doc/latest", None),
-    "matplotlib": ("https://matplotlib.org", None),
+    "matplotlib": ("https://matplotlib.org/stable/", None),
     "dask": ("https://docs.dask.org/en/latest", None),
     "cftime": ("https://unidata.github.io/cftime", None),
     "rasterio": ("https://rasterio.readthedocs.io/en/latest", None),