docs: use pull_request.head.sha for build (#5176)

.github/workflows/docs_build.yaml

@@ -20,6 +20,8 @@ jobs:
             -d '{"state":"pending","description":"Waiting for relevant checks to complete","context":"checks_integrated"}'
       - name: Checkout
         uses: actions/checkout@v3
+        with:
+          ref: ${{ github.event.pull_request.head.sha }}
       - name: Build
         uses: diplodoc-platform/docs-build-action@v3
         with: