ci(docs): use yarn cache

Author: malept

.github/workflows/docs.yml

@@ -17,6 +17,15 @@ jobs:
       - uses: actions/[email protected]
         with:
           node-version: 14.x
+      - name: Get yarn cache
+        id: yarn-cache
+        run: echo "::set-output name=dir::$(yarn cache dir)"
+      - uses: actions/[email protected]
+        with:
+          path: ${{ steps.yarn-cache.outputs.dir }}
+          key: ${{ runner.os }}-yarn-${{ hashFiles('**/package.json') }}
+          restore-keys: |
+            ${{ runner.os }}-yarn-
       - run: yarn
       - run: yarn docs
       - uses: docker://malept/gha-gh-pages:1.3.0