docs: Add more information to the HTML sidebar

Add a new sidebar template that creates a more RTD-like "fisheye" view of
the current place in the document hierarchy.  It is far from ideal, but
some readers may find it better for navigating through the documentation as
a whole.

Add some CSS trickery as well to make the table of contents less intrusive
when viewing the pages on a small screen.

Reviewed-by: Akira Yokosawa <akiyks@gmail.com>
Reviewed-by: David Gow <davidgow@google.com
Signed-off-by: Jonathan Corbet <corbet@lwn.net>

Documentation/conf.py

@@ -153,7 +153,7 @@ else:
     math_renderer = 'mathjax'
 
 # Add any paths that contain templates here, relative to this directory.
-templates_path = ['_templates']
+templates_path = ['sphinx/templates']
 
 # The suffix(es) of source filenames.
 # You can specify multiple suffix as a list of string:
@@ -328,6 +328,7 @@ if  html_theme == 'alabaster':
         'description': get_cline_version(),
         'page_width': '65em',
         'sidebar_width': '15em',
+        'fixed_sidebar': 'true',
         'font_size': 'inherit',
         'font_family': 'serif',
     }
@@ -345,7 +346,7 @@ html_use_smartypants = False
 
 # Custom sidebar templates, maps document names to template names.
 # Note that the RTD theme ignores this
-html_sidebars = { '**': ['searchbox.html', 'localtoc.html', 'sourcelink.html']}
+html_sidebars = { '**': ['searchbox.html', 'kernel-toc.html', 'sourcelink.html']}
 
 # about.html is available for alabaster theme. Add it at the front.
 if html_theme == 'alabaster':