fix doc CI script

Author: martinfouilleul

.github/workflows/build-doc-site.yml

@@ -17,6 +17,7 @@ jobs:
           python-version: 3.x
       - name: buid api doc
         run: |
+          pip install clang
           python scripts/gen_doc.py src/api.json doc/mkdocs
       - name: build site
         run: |