DOC: Enable Opensearch

see https://www.sphinx-doc.org/en/master/usage/configuration.html#confval-html_use_opensearch

Author: StefRe

doc/conf.py

@@ -458,7 +458,7 @@ def js_tag_with_cache_busting(js):
 
 # If true, an OpenSearch description file will be output, and all pages will
 # contain a <link> tag referring to it.
-html_use_opensearch = 'False'
+html_use_opensearch = 'https://matplotlib.org/stable'
 
 # Output file base name for HTML help builder.
 htmlhelp_basename = 'Matplotlibdoc'