use julia-actions/cache also for Documenter (#110)

Author: ranocha

.github/workflows/Documenter.yml

@@ -25,6 +25,7 @@ jobs:
         with:
           version: '1'
           show-versioninfo: true
+      - uses: julia-actions/cache@v1
       - uses: julia-actions/julia-buildpkg@v1
         env:
           PYTHON: ""