fix: Install elan in pipeline

.github/workflows/docs.yaml

@@ -28,6 +28,13 @@ jobs:
         pip install poetry
         poetry install --only doc
 
+    - name: install elan
+      run: |
+        set -o pipefail
+        curl -sSfL https://github.com/leanprover/elan/releases/download/v3.1.1/elan-x86_64-unknown-linux-gnu.tar.gz | tar xz
+        ./elan-init -y --default-toolchain none
+        echo "$HOME/.elan/bin" >> "${GITHUB_PATH}"
+
     # Build the book
     - name: Build the book
       run: |