Introduce GitHub Action to test resolvability of external links [DOC-…

…253] (#431)

Introduces [functionality introduced in
`hz-docs`](hazelcast/hz-docs#1382) to rest of
docs codebase.

.github/workflows/test-external-links.yml

@@ -0,0 +1,16 @@
+name: Test external links
+
+on:
+  workflow_dispatch:
+  schedule:
+    - cron: "0 12 * * 1" # Runs at 12:00, only on Monday
+
+jobs:
+  test-external-links:
+    runs-on: ubuntu-latest
+
+    steps:
+      - uses: actions/checkout@v4
+      - uses: hazelcast/hz-docs/.github/actions/test-external-links@main
+        with:
+          SLACK_WEBHOOK: ${{ secrets.SLACK_WEBHOOK_DOCS }}