add more documentation (#894)

docs/_static/query_transformations/multi_step_diagram.png

docs/_static/query_transformations/single_step_diagram.png

docs/getting_started/starter_example.md

docs/how_to/embeddings.md

docs/how_to/integrations.rst

docs/how_to/integrations/chatgpt_plugins.md

docs/how_to/integrations/using_with_langchain.md

docs/how_to/integrations/vector_stores.md

docs/how_to/optimizers.md

docs/how_to/output_parsing.md

docs/how_to/query_transformations.md

docs/index.rst

docs/reference/optimizers.rst

gpt_index/optimization/__init__.py