build(#16): Add caching to the CI

.github/workflows/ci.yml

@@ -1,10 +1,10 @@
 name: ci
-on:
+  on:
     push:
       branches:
         - master
         - main
-jobs:
+  jobs:
     deploy:
       runs-on: ubuntu-latest
       steps:
@@ -12,5 +12,9 @@ jobs:
         - uses: actions/setup-python@v2
           with:
             python-version: 3.x
+        - uses: actions/cache@v2
+          with:
+            key: ${{ github.ref }}
+            path: .cache
         - run: pip install mkdocs-material
         - run: mkdocs gh-deploy --force