Add ChangeLog to the docs (#6409)

CHANGELOG.md

CONTRIBUTING.md

docs/DOCS_README.md

docs/conf.py

docs/development/changelog.rst

docs/development/contributing.rst

docs/development/documentation.rst

docs/how_to/connector/modules.md

docs/how_to/connector/root.md

docs/how_to/integrations/vector_stores.md

docs/how_to/query_engine/advanced/query_transformations.md

docs/how_to/storage/customization.md

docs/index.rst

...ference/query/query_engines/sub_question_query_engine.rst

docs/requirements.txt

docs/use_cases/agents.md

docs/use_cases/queries.md