[3.9] bpo-40939: Use the new grammar for the grammar specification documentation (GH-19969) (#21641)

(We censor the heck out of actions and some other stuff using a custom "highlighter".)

(cherry picked from commit 72cabb2aa636272e608285f5a6ba83b62be9be4e)

Co-authored-by: Pablo Galindo <Pablogsal@gmail.com>
diff --git a/Doc/conf.py b/Doc/conf.py
index 12d74ea..bfb2a98 100644
--- a/Doc/conf.py
+++ b/Doc/conf.py
@@ -15,7 +15,7 @@
 
 extensions = ['sphinx.ext.coverage', 'sphinx.ext.doctest',
               'pyspecific', 'c_annotations', 'escape4chm',
-              'asdl_highlight']
+              'asdl_highlight', 'peg_highlight']
 
 
 doctest_global_setup = '''