docs: reconfigure the HTML left column

Use the html_sidebars directive to get a more useful set of links in the
left column.

Unfortunately, this is a no-op with the default RTD theme, but others
observe it.

Reviewed-by: David Vernet <void@manifault.com>
Acked-by: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>
Acked-by: Randy Dunlap <rdunlap@infradead.org>
Link: https://lore.kernel.org/r/20220927160559.97154-4-corbet@lwn.net


Signed-off-by: Jonathan Corbet <corbet@lwn.net>