chore(ci): bump GitHub Pages actions to v5 (#1190)

Combines dependabot's #1183 and #1184 into a single change.

`actions/upload-pages-artifact@v5` bumps its internal upload-artifact to
v7, so the producer and consumer must move together. Merging them
separately would run one real Pages deployment with a v5 upload feeding a
v4 deploy, since docs.yml is itself in the workflow's trigger paths.

Author: Kohei-Wada

.github/workflows/docs.yml

@@ -37,7 +37,7 @@ jobs:
 
       - name: Upload Pages artifact
         if: github.ref == 'refs/heads/main' && github.event_name == 'push'
-        uses: actions/upload-pages-artifact@v4
+        uses: actions/upload-pages-artifact@v5
         with:
           path: site/
 
@@ -58,4 +58,4 @@ jobs:
     steps:
       - name: Deploy to GitHub Pages
         id: deployment
-        uses: actions/deploy-pages@v4
+        uses: actions/deploy-pages@v5