rename GPT Index -> LlamaIndex in docs (#550)

CITATION.cff

CONTRIBUTING.md

README.md

docs/conf.py

docs/gallery/app_showcase.md

docs/getting_started/starter_example.md

docs/guides/index_guide.md

docs/guides/primer.md

docs/guides/usage_pattern.md

docs/guides/use_cases.md

docs/how_to/composability.md

docs/how_to/cost_analysis.md

docs/how_to/custom_llms.md

docs/how_to/custom_prompts.md

docs/how_to/data_connectors.md

docs/how_to/embeddings.md

docs/how_to/update.md

docs/how_to/using_with_langchain.md

docs/how_to/vector_stores.md

docs/index.rst