docs/doc-guide: Add footnote on Inkscape for better images in PDF documents

With kernel releases v5.18 and later, if you have Inkscape, images
embedded in PDF documents are of vector graphics, not the raster
images as are the case with pre-v5.18 releases.
Even with pre-5.18 releases, having Inkscape would improve images
converted from some of SVG files which are not fully covered by the
limited capability of rsvg-convert(1) [1].

Add a footnote mentioning the expected improvements of such images.

Link: https://gitlab.gnome.org/GNOME/librsvg#non-goals-of-librsvg

 [1]
Signed-off-by: Akira Yokosawa <akiyks@gmail.com>
Link: https://lore.kernel.org/r/81cddbde-bc28-bec1-fca4-3c8fe2df502f@gmail.com


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