fix: use rsvg-convert as the conversion backend

The auxiliary pdf build requires SVG files to be converted to pdf files.
Use libRSVG as a light weight alternative to inkscape, and ensure it's
installed in the build environment.

Change-Id: I1fdb05fb2701fc28a04e210d5928f4387ca08613
Signed-off-by: Harrison Mutai <harrison.mutai@arm.com>
diff --git a/docs/conf.py b/docs/conf.py
index 345f53b..1549032 100644
--- a/docs/conf.py
+++ b/docs/conf.py
@@ -26,7 +26,7 @@
     "myst_parser",
     "sphinx.ext.autosectionlabel",
     "sphinxcontrib.plantuml",
-    "sphinxcontrib.inkscapeconverter",
+    "sphinxcontrib.rsvgconverter",
 ]
 
 # Add any paths that contain templates here, relative to this directory.