Add validation-only docs workflow for non-main branches

.forgejo/workflows/validate-docs.yml

@@ -0,0 +1,28 @@
+name: Validate Docs
+
+on:
+  push:
+    branches:
+      - '**'
+      - '!main'
+
+jobs:
+  validate-docs:
+    runs-on: docker
+
+    steps:
+      - name: Checkout
+        uses: actions/checkout@v4
+
+      - 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"