Add GitHub Action for mdbook

BUG=b:199688855
TEST=run at personal account

Change-Id: I7e44424deccf64be5fdf0d14d24109c3245e37c1
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/crosvm/+/3159885
Reviewed-by: Dennis Kempin <denniskempin@google.com>
Tested-by: kokoro <noreply+kokoro@google.com>
Commit-Queue: Keiichi Watanabe <keiichiw@chromium.org>

.github/workflows/gh-pages.yml

@@ -0,0 +1,28 @@
+name: github pages
+
+on:
+  push:
+    branches:
+      - main
+  pull_request:
+
+jobs:
+  deploy:
+    runs-on: ubuntu-20.04
+    concurrency:
+      group: ${{ github.workflow }}-${{ github.ref }}
+    steps:
+      - uses: actions/checkout@v2
+
+      - name: Setup mdBook
+        uses: peaceiris/actions-mdbook@v1
+        with:
+          mdbook-version: 'latest'
+      - run: cargo install mdbook-mermaid
+      - run: mdbook build docs/book/
+
+      - name: Deploy
+        uses: peaceiris/actions-gh-pages@v3
+        with:
+          github_token: ${{ secrets.GITHUB_TOKEN }}
+          publish_dir: ./docs/book/book