Prompt refactor (#7319)

CHANGELOG.md

docs/api_reference/prompts.rst

docs/core_modules/model_modules/llms/usage_custom.md

docs/core_modules/model_modules/prompts.md

.../core_modules/query_modules/chat_engines/usage_pattern.md

...modules/query_modules/structured_outputs/output_parser.md

...torials/question_and_answer/terms_definitions_tutorial.md

...ustomization/llms/SimpleIndexDemo-Huggingface_camel.ipynb

...omization/llms/SimpleIndexDemo-Huggingface_stablelm.ipynb

docs/examples/customization/prompts/chat_prompts.ipynb

docs/examples/customization/prompts/completion_prompts.ipynb

docs/examples/output_parsing/GuardrailsDemo.ipynb

docs/examples/output_parsing/LangchainOutputParserDemo.ipynb

docs/examples/query_engine/ensemble_query_engine.ipynb

docs/examples/vector_stores/SimpleIndexDemoLlama-Local.ipynb

examples/paul_graham_essay/TestEssay.ipynb

examples/test_wiki/TestNYC-Benchmark-GPT4.ipynb

experimental/classifier/utils.py

llama_index/__init__.py

llama_index/agent/context_retriever_agent.py