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>

.readthedocs.yaml

@@ -15,6 +15,7 @@ build:
     python: "3.10"
   apt_packages:
     - plantuml
+    - librsvg2-bin
   jobs:
     post_create_environment:
       - pip install poetry=="1.3.2"

docs/conf.py

@@ -26,7 +26,7 @@ extensions = [
     "myst_parser",
     "sphinx.ext.autosectionlabel",
     "sphinxcontrib.plantuml",
-    "sphinxcontrib.inkscapeconverter",
+    "sphinxcontrib.rsvgconverter",
 ]
 
 # Add any paths that contain templates here, relative to this directory.