Update deployment docs to avoid relying on internal get_fast_api_app

[xuanyang15]

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: google/adk-python#4560

[meichuanyi]

Hi! I'd like to work on this issue to update the deployment docs. I'll create a PR that shows how to create a FastAPI object explicitly instead of relying on the internal method. Thanks!