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 <[email protected]>
Reviewed-by: David Gow <[email protected]
Signed-off-by: Jonathan Corbet <[email protected]>

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