move Document object, add Document to top-level (#119)

docs/how_to/insert.md

examples/paul_graham_essay/InsertDemo.ipynb

experimental/classifier/TitanicModel.ipynb

gpt_index/__init__.py

gpt_index/readers/__init__.py

gpt_index/readers/base.py

gpt_index/readers/faiss.py

gpt_index/readers/file.py

gpt_index/readers/google/gdocs.py

gpt_index/readers/mongo.py

gpt_index/readers/notion.py

gpt_index/readers/pinecone.py

gpt_index/readers/schema/__init__.py

gpt_index/readers/schema/base.py

gpt_index/readers/slack.py

gpt_index/readers/weaviate.py

gpt_index/readers/wikipedia.py

gpt_index/schema.py

tests/indices/embedding/test_base.py

tests/indices/keyword_table/test_base.py