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/.readthedocs.yaml b/.readthedocs.yaml
index e3a7ebf..7b6a1f5 100644
--- a/.readthedocs.yaml
+++ b/.readthedocs.yaml
@@ -15,6 +15,7 @@
     python: "3.10"
   apt_packages:
     - plantuml
+    - librsvg2-bin
   jobs:
     post_create_environment:
       - pip install poetry=="1.3.2"