Add workflow file to index docs into Plain (#379)

Author: mattvagni

.github/workflows/index-sitemap.yml

@@ -0,0 +1,24 @@
+name: Index docs
+
+on:
+  schedule:
+    - cron: "0 */3 * * *"  
+
+jobs:
+  lint:
+    name: Lint
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v2
+
+      - uses: actions/setup-node@v3
+        with:
+          node-version: '18'
+
+      - name: Install CLI
+        run: npm install -g @team-plain/cli@latest
+        
+      - name: Index Docs
+        run: plain index-sitemap https://mintlify.com/docs/sitemap.xml
+        env:
+          PLAIN_API_KEY: ${{ secrets.PLAIN_API_KEY }}