[sphinx] replace deprecated 'master_doc' with 'root_doc' (#9600)

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

doc/conf.py

@@ -106,8 +106,8 @@
 # The encoding of source files.
 # source_encoding = 'utf-8-sig'
 
-# The master toctree document.
-master_doc = "index"
+# The root toctree document.
+root_doc = "index"
 
 # General information about the project.
 project = "Pylint"