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

Documentation/doc-guide/sphinx.rst

@@ -132,7 +132,8 @@ format-specific subdirectories under ``Documentation/output``.
 To generate documentation, Sphinx (``sphinx-build``) must obviously be
 installed. For prettier HTML output, the Read the Docs Sphinx theme
 (``sphinx_rtd_theme``) is used if available. For PDF output you'll also need
-``XeLaTeX`` and ``convert(1)`` from ImageMagick (https://www.imagemagick.org).
+``XeLaTeX`` and ``convert(1)`` from ImageMagick
+(https://www.imagemagick.org).\ [#ink]_
 All of these are widely available and packaged in distributions.
 
 To pass extra options to Sphinx, you can use the ``SPHINXOPTS`` make
@@ -152,6 +153,10 @@ The Sphinx theme can be overridden by using the ``DOCS_THEME`` make variable.
 
 To remove the generated documentation, run ``make cleandocs``.
 
+.. [#ink] Having ``inkscape(1)`` from Inkscape (https://inkscape.org)
+	  as well would improve the quality of images embedded in PDF
+	  documents, especially for kernel releases 5.18 and later.
+
 Writing Documentation
 =====================