Change `AgentWorkflow` to `FunctionAgent` in documentation. (#18042)

docs/docs/getting_started/starter_example.md

docs/docs/getting_started/starter_example_local.md

docs/docs/understanding/agent/human_in_the_loop.md

docs/docs/understanding/agent/index.md

docs/docs/understanding/agent/streaming.md

docs/docs/understanding/agent/tools.md

...-index-core/llama_index/core/agent/workflow/base_agent.py

...e/llama_index/core/agent/workflow/multi_agent_workflow.py