ci: add sync CI

See #86 (comment)

.github/workflows/sync.yaml

@@ -0,0 +1,32 @@
+name: Auto Sync
+
+on:
+  schedule:
+    - cron: '0 0 * * *' # At 00:00.  https://crontab.guru/
+  workflow_dispatch: # on button click
+
+jobs:
+  sync:
+    runs-on: ubuntu-latest
+    permissions:
+      contents: write # 'write' access to repository contents
+    steps:
+      - name: Install git-filter-repo
+        run: |
+          sudo apt-get update
+          sudo apt-get install -y git-filter-repo
+
+      - name: Checkout upstram
+        uses: actions/checkout@v2
+        with:
+          repository: slidevjs/slidev
+          fetch-depth: 0
+
+      - name: Extract docs
+        run: git filter-repo --subdirectory-filter docs --force
+
+      - name: Push changes
+        uses: ad-m/[email protected]
+        with:
+          github_token: ${{ secrets.GITHUB_TOKEN }}
+          branch: refs/heads/upstream