chore(deps): update actions/cache action to v4 (#27)

Co-authored-by: renovate[bot] <29139614+renovate[bot]@users.noreply.github.com>

Author: renovate[bot]

.github/workflows/docs.yaml

@@ -38,7 +38,7 @@ jobs:
         run: |
           echo "cache_id=$(date --utc '+%V')" >> $GITHUB_ENV
       - name: handle cache
-        uses: actions/cache@v3
+        uses: actions/cache@v4
         with:
           key: mkdocs-material-${{ env.cache_id }}
           path: .cache