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

Documentation/conf.py

@@ -369,7 +369,8 @@ html_static_path = ['sphinx-static']
 html_use_smartypants = False
 
 # Custom sidebar templates, maps document names to template names.
-#html_sidebars = {}
+# Note that the RTD theme ignores this.
+html_sidebars = { '**': ['searchbox.html', 'localtoc.html', 'sourcelink.html']}
 
 # Additional templates that should be rendered to pages, maps page names to
 # template names.