Fixes errors encountered when handling SVG graphics, unicode characters,
and deeply nested lists (i.e. in the change log) with the `latexpdf`
docs build. Adds `sphinxcontrib-svg2pdfconverter` to allow embedding SVG
images into PDF files; changes the LaTeX engine to XeLaTex to provide
wider support for unicode characters (see [1] for more details); and
increases the maximum list depth.

[1] https://www.sphinx-doc.org/en/master/usage/configuration.html#confval-latex_engine



Change-Id: I2ee265d301f6822bae7aa6dfa3a8bfcf070076d3
Signed-off-by: Harrison Mutai <harrison.mutai@arm.com>