docs/sphinx: More depth in the rtd sidebar toc

docs/sphinx: More depth in the rtd sidebar toc

We love to nest our documenation for good structure, but that means
the table of contents needs to keep up or you can't navigate them.

Realized this trying to find the drm property documentation, which
with some shuffling around disappeared. Why I didn't realize we can do
this earlier, no idea.

Since the relevant parts of the toc are only loaded if you're in the
right .html file there's no harm in going all the way to unlimited.

Note that this has no impact on the alabaster theme (which has a much
simpler sidebar toc which doesn't show the entire hierarchy, only
what's in the local rendered file) nor on the various :toctree:
rendered inline in the output.

Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Cc: Jonathan Corbet <corbet@lwn.net>
Cc: linux-doc@vger.kernel.org
Link: https://lore.kernel.org/r/20221108115707.1232621-1-daniel.vetter@ffwll.ch
Signed-off-by: Jonathan Corbet <corbet@lwn.net>