docs: enable fixed_sidebar

docs/conf.py

@@ -17,6 +17,7 @@ html_theme = 'alabaster'
 html_theme_options = {
     'font_family': "Georgia, serif",
     'head_font_family': "Georgia, serif",
+    'fixed_sidebar': True,
 }
 htmlhelp_basename = 'mitogendoc'
 intersphinx_mapping = {'python': ('https://docs.python.org/3', None)}