[LNT] Fix sidebar

[kpdev42]

Summary

This patch fixes the sidebar.
Here is the documentation https://www.sphinx-doc.org/en/master/usage/configuration.html#confval-html_sidebars

[thopre]

So this is to be compatible with sphinx 2.0? LGTM

docs/conf.py

 # using the given strftime format.
 html_last_updated_fmt = '%Y-%m-%d'
 
 # If true, SmartyPants will be used to convert quotes and dashes to
 # typographically correct entities.
 #html_use_smartypants = True
 
 # Custom sidebar templates, maps document names to template names.
-html_sidebars = {'index': 'indexsidebar.html'}
+html_sidebars = {'index': ['indexsidebar.html']}
 
 # Additional templates that should be rendered to pages, maps page names to
 # template names.
 html_additional_pages = {'index': 'index.html'}
 
 # If false, no module index is generated.
 #html_use_modindex = True