Add caching (julia-actions/cache) to Documenter.yml workflow

Author: abhro

.github/workflows/Documenter.yml

@@ -19,6 +19,7 @@ jobs:
       - uses: julia-actions/setup-julia@v2
         with:
           version: '1'
+      - uses: julia-actions/cache@v2
       - name: Install dependencies
         shell: julia --color=yes --project=docs {0}
         run: 'using Pkg; Pkg.instantiate()'