new notebooks and openai upgrade

docs/00-introduction.ipynb

docs/01-save-load-from-file.ipynb

docs/02-dynamic-routes.ipynb

docs/03-basic-langchain-agent.ipynb

docs/05-local-execution.ipynb

docs/06-threshold-optimization.ipynb

docs/encoders/fastembed.ipynb

docs/encoders/huggingface.ipynb

docs/encoders/openai-embed-3.ipynb

semantic_router/__init__.py

semantic_router/encoders/openai.py