Update docs: Update imports for LangChain provider

[soos3d]

Update imports for LangChain provider. The current ones are deprecated and result in those warnings:

[WARNING]: Importing from "langchain/chat_models/openai" is deprecated.

Instead, please add the "@langchain/openai" package to your project with e.g.

    $ npm install @langchain/openai

and import from "@langchain/openai".

This will be mandatory after the next "langchain" minor version bump to 0.2.
[WARNING]: Importing from "langchain/schema/output_parser" is deprecated.

Instead, please import from "@langchain/core/output_parsers".

This will be mandatory after the next "langchain" minor version bump to 0.2.
[WARNING]: Importing from "langchain/prompts" is deprecated.

Instead, please import from "@langchain/core/prompts".

This will be mandatory after the next "langchain" minor version bump to 0.2.