docs/sphinx: Fix TOC scroll hack for the home page

When on the documentation home page, there won't be any ".current"
element since no entry from the TOC was selected yet. That results in a
javascript error. Fix that by only trying to set the scrollTop if we
have matches for current entries.

Signed-off-by: Gustavo Sousa <gustavo.sousa@intel.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>
Link: https://lore.kernel.org/r/20240123162157.61819-2-gustavo.sousa@intel.com

docs: Use HTML comments for the kernel-toc SPDX line

This line was initially placed in {# jinja2 comments #}, but that led to an
"invalid token" complaint from spdxcheck.py. Rather than fix the script
for a usage we'll likely never see anywhere else, just switch to an HTML
comment, which spdxcheck.py thinks is fine.

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

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>