ci: add workflow to .forgejo directory

.forgejo/workflows/validate-docs.yaml

@@ -0,0 +1,38 @@
+name: Validate Docs
+
+on:
+  push:
+    branches:
+      - '**'
+      - '!main'
+
+jobs:
+  validate-docs:
+    runs-on: docs-host
+
+    steps:
+      - name: Checkout Repository
+        uses: actions/checkout@v4
+
+      - name: Render PlantUML diagrams to SVG
+        run: |
+          mkdir -p docs/images
+          docker run --rm \
+            -v "$PWD:/work" \
+            -w /work \
+            ghcr.io/plantuml/plantuml \
+            -tsvg docs/architecture/workspace/*.puml
+          cp docs/architecture/workspace/*.svg docs/images/
+
+      - name: Build MkDocs Site
+        run: |
+          docker run --rm \
+            -v "$PWD:/docs" \
+            squidfunk/mkdocs-material:9 \
+            build
+
+      - name: Validate Expected Output
+        run: |
+          test -f site/index.html
+          test -d site/architecture
+          echo "MkDocs validation passed"