lld docs config: Use a list key in html_sidebars

Otherwise the docs-lld-html target fails to build using recent Sphinx
with the following not very helpful error message:

An error happened in rendering the page index.
Reason: TemplateNotFound()

It turns out the values in the html_sidebars dictionary always need to be lists
now. See sphinx-doc/sphinx#6186

lld/docs/conf.py

@@ -134,7 +134,7 @@
 #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.