Llama index readers gitbook (#16862)

...tegrations/readers/llama-index-readers-gitbook/.gitignore

...ex-integrations/readers/llama-index-readers-gitbook/BUILD

...grations/readers/llama-index-readers-gitbook/CHANGELOG.md

...integrations/readers/llama-index-readers-gitbook/Makefile

...ntegrations/readers/llama-index-readers-gitbook/README.md

...a-index-readers-gitbook/llama_index/readers/gitbook/BUILD

...x-readers-gitbook/llama_index/readers/gitbook/__init__.py

...index-readers-gitbook/llama_index/readers/gitbook/base.py

...ers-gitbook/llama_index/readers/gitbook/gitbook_client.py

...egrations/readers/llama-index-readers-gitbook/poetry.lock

...ations/readers/llama-index-readers-gitbook/pyproject.toml

...egrations/readers/llama-index-readers-gitbook/tests/BUILD

...ons/readers/llama-index-readers-gitbook/tests/__init__.py

.../readers/llama-index-readers-gitbook/tests/fixtures/BUILD

...readers-gitbook/tests/fixtures/page_content_response.json

...-index-readers-gitbook/tests/fixtures/pages_response.json

...-index-readers-gitbook/tests/fixtures/space_response.json

.../llama-index-readers-gitbook/tests/test_gitbook_client.py

...index-readers-gitbook/tests/test_simple_gitbook_reader.py