Add caching to CI docs workflow (#971)

Author: abhro

.github/workflows/ci.yml

@@ -72,6 +72,7 @@ jobs:
       - uses: julia-actions/setup-julia@v2
         with:
           version: '1'
+      - uses: julia-actions/cache@v2
       - uses: julia-actions/julia-buildpkg@v1
       - uses: julia-actions/julia-docdeploy@v1
         env: