fix(tpl): resolve broken assets on GitHub Pages subdirectory deployments

Add `actions/configure-pages` step to all template build workflows and
pass the detected base_url to `hugo --baseURL`, so sites deployed to
subdirectory paths (e.g. user.github.io/repo-name/) generate correct
asset URLs. Also fix leading-slash relURL calls in slides layout that
would ignore the baseURL path component.

templates/documentation/.github/workflows/build.yml

@@ -112,13 +112,17 @@ jobs:
         restore-keys: |
           ${{ runner.os }}-hugo-resources-
 
+    - name: Setup Pages
+      id: pages
+      uses: actions/configure-pages@v5
+
     - name: Build with Hugo
       env:
         HUGO_ENVIRONMENT: production
         HUGO_BLOX_LICENSE: ${{ secrets.HUGO_BLOX_LICENSE }}
       run: |
         echo "Hugo Cache Dir: $(hugo config | grep cachedir)"
-        hugo --minify
+        hugo --minify --baseURL "${{ steps.pages.outputs.base_url }}/"
 
     - name: Generate Pagefind search index (if applicable)
       run: |