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 https://github.com/sphinx-doc/sphinx/issues/6186