Simplify documenation step

.github/workflows/ci-nix.yaml

@@ -278,25 +278,10 @@ jobs:
           accept-flake-config = true
           log-lines = 1000
 
-    - name: Set up and use the "ci" devShell
-      uses: nicknovitski/nix-develop@v1
-      with:
-        arguments: ".#ci"
-
-    # Technically, we don't need this, given we're in a Nix shell;
-    # but we will keep it for the caching.
-    - name: 🚧 Setup Node.js
-      uses: actions/setup-node@v4
-      with:
-        node-version: 18
-        cache: 'yarn'
-        cache-dependency-path: docs/yarn.lock
-
     - name: ❓ Test API reference
       working-directory: docs
       run: |
-        yarn
-        yarn validate
+        nix develop .#ci --command 'yarn; yarn validate'
 
     - name: 📥 Download benchmark results
       uses: actions/download-artifact@v4