Add the "Fork me on GitHub" banner

Also show the GitHub star count.

Author: Mariatta

conf.py

@@ -145,7 +145,12 @@
 # further.  For a list of options available for each theme, see the
 # documentation.
 #
-# html_theme_options = {}
+html_theme_options = {
+    'github_user': 'asyncio-doc',
+    'github_repo': 'asyncio-doc',
+    'github_banner': True,
+    'github_type': 'star',
+}
 
 # Add any paths that contain custom themes here, relative to this directory.
 # html_theme_path = []