fix(docs): add plantuml as a dependency

This wasn't listed on the web interface configuration. Perhaps it came
preloaded. Anyway, it's needed for diagrams. Add it back.

Signed-off-by: Boyan Karatotev <boyan.karatotev@arm.com>
Change-Id: I20c1eb0e8d5abaa3533169dd9704cbd3b0eb06a5
(cherry picked from commit 65982a94ef113d5d652d8e1a521b219be75fca42)
Signed-off-by: Yann Gautier <yann.gautier@st.com>
diff --git a/.readthedocs.yaml b/.readthedocs.yaml
index 3663f37..6e6fa84 100644
--- a/.readthedocs.yaml
+++ b/.readthedocs.yaml
@@ -13,6 +13,8 @@
   os: ubuntu-22.04 # Ubuntu Jammy LTS
   tools:
     python: "3.10"
+  apt_packages:
+    - plantuml
 
 python:
   install: