Add a docs/config-reference.md page documenting the full YAML schema

[himanshu231204]

Description

Users currently have to read the source code or existing \config.yaml\ examples to understand all available configuration options. A dedicated config reference page would make the project more accessible.

What to Add

Create \docs/config-reference.md\ documenting the complete YAML schema:

For each config section (dataset, llm, retriever, metrics, report, corpus, global), document:

• All available fields
• Their types and default values
• Whether they are required or optional
• Example values
• Brief description

Files to Look At

• \openagent_eval/config/models.py\ — the Pydantic models defining the schema
• \config.yaml\ — example config
• \mkdocs.yml\ — to register the new page

Good First Issue ✅

A documentation task that builds understanding of the full configuration system.