Add deploy CI script

.github/workflows/docs.yaml

@@ -0,0 +1,38 @@
+on:
+  push:
+    branches:
+      - main
+
+jobs:
+  changes:
+    runs-on: ubuntu-latest
+    outputs:
+      dirs: ${{ steps.filter.outputs.changes }}
+    steps:
+      - uses: actions/checkout@v2
+      - uses: dorny/paths-filter@v2
+        id: filter
+        with:
+          filters: |
+            docs: docs/**
+  deploy:
+    needs: [changes]
+    if: ${{ !contains(needs.changes.outputs.dirs, '[]') }}
+    runs-on: ubuntu-latest
+    concurrency:
+      group: ${{ github.workflow }}-${{ github.ref }}
+    steps:
+      - uses: actions/checkout@v2
+      - name: Setup mdBook
+        uses: peaceiris/actions-mdbook@v1
+        with:
+          mdbook-version: 'latest'
+      - name: Build mdbook
+        working-directory: ./docs
+        run: mdbook build
+      - name: Deploy
+        uses: peaceiris/actions-gh-pages@v3
+        if: ${{ github.ref == 'refs/heads/main' }}
+        with:
+          github_token: ${{ secrets.GITHUB_TOKEN }}
+          publish_dir: ./docs/book