docs: sphinx/kfigure.py: Add check of 'dot -Tpdf'

To prevent any regression on existing build systems, limit the
fallback of converting DOT -> raster PDF only when both of the
following conditions are met.

 o dot(1) doesn't support -Tpdf
 o rsvg-convert(1) is not found

While we are here, add kernellog.verbose messages related to
rsvg-convert, 'dot -Tpdf', and 'dot -Tsvg' commands.

Suggested-by: Mauro Carvalho Chehab <mchehab@kernel.org>
Signed-off-by: Akira Yokosawa <akiyks@gmail.com>
Cc: Jonathan Corbet <corbet@lwn.net>
Link: https://lore.kernel.org/r/e76f61e1-7366-ba00-b119-8ea6a2499861@gmail.com


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