add copy codeblock button

Apply suggestions from code review

Co-authored-by: Danny <Rapptz@users.noreply.github.com>

Change to icon, change according to slice's review

docs/conf.py

@@ -320,5 +320,6 @@ texinfo_documents = [
 
 def setup(app):
   app.add_js_file('custom.js')
+  app.add_js_file('copy.js')
   if app.config.language == 'ja':
     app.config.intersphinx_mapping['py'] = ('https://docs.python.org/ja/3', None)