Update deployment docs to avoid relying on internal get_fast_api_app

In google/adk-python PR #4560, @olliefr suggested that the documentation (e.g., Cloud Run and GKE deployment examples) should not rely on the internal method 'get_fast_api_app'. If this interface is not public, the documentation should show users how to create a FastAPI object explicitly. Ref: https://github.com/google/adk-python/pull/4560