chore(ci): add GitHub action for showcase testing (#6002)

.github/workflows/showcase-test.yml

@@ -0,0 +1,26 @@
+name: Showcase Tests
+
+on:
+  pull_request:
+    branches:
+      - main
+    paths:
+      - website/src/data/**
+
+jobs:
+  validate-config:
+    name: Validate Showcase Config
+    timeout-minutes: 30
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v2
+      - name: Use Node.js ${{ matrix.node }}
+        uses: actions/setup-node@v2
+        with:
+          node-version: ${{ matrix.node }}
+          cache: yarn
+      - name: Installation
+        run: yarn
+      - name: Test
+        run: yarn test website/src/data/__tests__/user.test.ts
+  # TODO another job to optimize the images, see https://github.com/facebook/docusaurus/issues/5980