cache to speed up doc build?

.github/workflows/documentation.yml

@@ -18,6 +18,7 @@ jobs:
       - uses: julia-actions/setup-julia@v2
         with:
           version: '1'
+      - uses: julia-actions/cache@v2
       - name: Install dependencies
         run: julia --project=docs/ -e 'using Pkg; Pkg.develop(PackageSpec(path=pwd())); Pkg.instantiate()'
       - name: Build and deploy without coverage