Docs: Create Integration Guides for LangChain & LlamaIndex

[c21051997]

Is your feature request related to a problem? Please describe.
Many potential users are already using popular frameworks like LangChain and LlamaIndex. It's not immediately obvious how to integrate ragscope into a project built with these tools.

Describe the solution you'd like
Create a new "Integrations" section in the documentation (or a new docs/ folder) with specific, copy-paste-ready examples of how to:

1. Wrap a LangChain Chain so it can be evaluated by ragscope.
2. Wrap a LlamaIndex QueryEngine so it can be evaluated by ragscope.

Describe alternatives you've considered
We could build custom wrapper classes, but simple documentation guides are a much better and easier first step.

Additional context
This would significantly lower the barrier to entry for a large portion of the target user base.