Enabled opensearch for Cabal documentation

doc/conf.py

@@ -94,7 +94,7 @@ html_theme_options = {
 # If true, an OpenSearch description file will be output, and all pages will
 # contain a <link> tag referring to it.  The value of this option must be the
 # base URL from which the finished HTML is served.
-#html_use_opensearch = ''
+html_use_opensearch = 'https://cabal.readthedocs.io/en/stable'
 
 # This is the file name suffix for HTML files (e.g. ".xhtml").
 #html_file_suffix = None