ci: Add build docs run section to Post-release workflow

.github/workflows/post_release.yml

@@ -17,6 +17,7 @@ jobs:
         uses: actions/setup-node@v1
         with:
           node-version: 12
+      - name: Build docs
         run: |
           yarn --cwd ./docs install --frozen-lockfile
           yarn --cwd ./docs build