fix: make new check-removed-urls workflow reusable (#624)

Author: rajannpatel

.github/workflows/check-removed-urls.yml

@@ -25,6 +25,11 @@ jobs:
           repository: ${{ github.event.pull_request.base.repo.full_name }}
           fetch-depth: 0
           path: base
+      - name: Checkout sphinx-stack
+        uses: actions/checkout@v5
+        with:
+          repository: canonical/sphinx-stack
+          path: sphinx-stack
       - uses: actions/setup-python@v6
       - name: Build docs
         run: |
@@ -49,4 +54,4 @@ jobs:
               | sort > "${urls_file}"
           done
       - name: Compare URLs
-        run: python3 compare/docs/_dev/check_removed_urls.py
+        run: python3 sphinx-stack/docs/_dev/check_removed_urls.py