validate dependabot configuration

Borrowed from Ubuntu
(ubuntu/authd@3f9df8f)

Sadly, this won't check our existing config unless I make a dummy
update.

.github/dependabot.yml

@@ -13,3 +13,4 @@ updates:
     schedule:
       # Check for updates to GitHub Actions every week
       interval: "weekly"
+

.github/workflows/dependabot.yml

@@ -0,0 +1,26 @@
+# copied from https://github.com/ubuntu/authd/commit/3f9df8f21d952cd33fd44d3834d0edeec1f5766f
+
+name: Dependabot rules validation
+
+permissions:
+  pull-requests: write
+
+on:
+  pull_request:
+    paths:
+      - '.github/dependabot.yml'
+
+jobs:
+  validate:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v4
+
+      - uses: marocchino/validate-dependabot@v3
+        id: validate
+
+      - uses: marocchino/sticky-pull-request-comment@v2
+        if: always()
+        with:
+          header: validate-dependabot
+          message: ${{ steps.validate.outputs.markdown }}