Change CI trigger to PR (forks), add build-book job

.github/workflows/ci.yml

@@ -1,6 +1,6 @@
-name: ci-push
+name: ci-pr
 on: 
-  push:
+  pull_request:
     branches-ignore:
       - staging
       - production
@@ -113,3 +113,20 @@ jobs:
         run: cd clang && ctest -V -C Debug
       - name: test release
         run: cd clang && ctest -V -C Release
+  build-book:
+      runs-on: ubuntu-latest
+      steps:
+      - uses: actions/checkout@v4
+        with:
+          fetch-depth: 0
+      - name: init
+        run: |
+          url="https://github.com/rust-lang/mdBook/releases/download/v0.4.47/mdbook-v0.4.47-x86_64-unknown-linux-gnu.tar.gz"
+          mkdir mdbook
+          curl -sSL $url | tar -xz --directory=./mdbook
+          echo `pwd`/mdbook >> $GITHUB_PATH
+      - name: build book
+        run: |
+          cd guide
+          mdbook build
+          ls book