[DOC]: update the endpoints in langchain and llamaindex notebooks

[sosahi]

How would you describe the priority of this documentation request

None

Please provide a link or source to the relevant docs

https://github.com/NVIDIA/nv-ingest/tree/main/examples

Describe the problems in the documentation

update the notebooks to use langchain-nvidia-ai-endpoints instead of langchain-openai. In a similar way, the llamaindex notebook can make use of the connectors for NIMs.

(Optional) Propose a correction or improvement

No response