Revert "remove build-docs version matrix. Just use variable once"

This reverts commit 25eeeed.

Author: schloerke

.github/workflows/build-docs.yaml

@@ -13,6 +13,10 @@ on:
 jobs:
   build-docs:
     runs-on: ubuntu-latest
+    strategy:
+      matrix:
+        python-version: ["3.13"]
+      fail-fast: false
 
     steps:
       - uses: actions/checkout@v4
@@ -22,7 +26,7 @@ jobs:
       - name: Setup py-shiny
         uses: ./.github/py-shiny/setup
         with:
-          python-version: "3.13"
+          python-version: ${{ matrix.python-version }}
 
       # =====================================================
       # API docs