feat: re-generate plantuml images in CI

.github/workflows/pr-plantuml-generate.yaml

@@ -0,0 +1,30 @@
+name: PR verify-plantuml
+
+on:
+  pull_request:
+    types: [opened, edited, synchronize, reopened]
+
+# Remove all permissions from GITHUB_TOKEN except metadata.
+permissions: {}
+
+jobs:
+  plantuml-generate-check:
+    name: Image generation reproducible test
+    runs-on: ubuntu-latest
+    container:
+      image: plantuml/plantuml:1.2023
+    steps:
+      - uses: actions/checkout@f43a0e5ff2bd294095638e18286ca9a3d1956744 # tag=v3.6.0
+      - name: "Prerequisite: install git"
+        run: apt-get -qq update && apt-get install -qq git # git is required for the diff, it isn't contained in the plantuml image
+      - name: Generate Plantuml Images
+        run:  java -jar /opt/plantuml.jar ./docs/book/**/*.plantuml && java -jar /opt/plantuml.jar ./docs/proposals/**/*.plantuml
+      - name: Check if anything changed
+        run: |
+          if git diff --quiet ./docs/; then
+            echo "No changes from image re-generation."
+          else
+            echo "Re-generating images resulted in changes:"
+            git status --porcelain
+            exit 1
+          fi