fix(ci): use linux-il2cpp image, no base tag exists for Unity 6

The ubuntu-6000.0.73f1-base-3 tag does not exist. Only platform-specific
images are published for Unity 6000.0.73f1: android, webgl, linux-il2cpp,
mac-mono. Switch to ubuntu-6000.0.73f1-linux-il2cpp-3.2.2, which includes
the full editor with all managed DLLs.

Also add mkdir -p lib before docker cp since the cache step may not have
restored the directory on a cache miss.

https://claude.ai/code/session_01N4YoJdJc2JDaoAbk5Nt8vf

Author: claude

.github/workflows/documentation.yml

@@ -36,7 +36,8 @@ jobs:
       - name: Extract Unity managed DLLs from game-ci image
         if: steps.cache-unity-dlls.outputs.cache-hit != 'true'
         run: |
-          docker create --name unity-editor unityci/editor:ubuntu-6000.0.73f1-base-3
+          mkdir -p lib
+          docker create --name unity-editor unityci/editor:ubuntu-6000.0.73f1-linux-il2cpp-3.2.2
           docker cp unity-editor:/opt/unity/Editor/Data/Managed/UnityEngine lib/
           docker rm unity-editor