[LNT] Fix sidebar

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

OS Laboratory. Huawei Russian Research Institute. Saint-Petersburg

Reviewed By: thopre

Differential Revision: https://reviews.llvm.org/D115701

Author: Pavel Kosov

docs/conf.py

@@ -214,7 +214,7 @@
 #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.