docs: Add documentation for metrics.collections API

[sanjeed5]

Summary

This PR updates the documentation for metrics to showcase the new ragas.metrics.collections API as the primary recommended approach, while preserving legacy API documentation for backward compatibility.

Changes

Metrics

• Faithfulness metric documentation updated with collections API examples
• AnswerAccuracy metric documentation updated with collections API examples (tested and verified)
• AnswerCorrectness
• AnswerRelevancy
• AnswerSimilarity
• BleuScore
• ContextEntityRecall
• ContextPrecision (including WithReference, WithoutReference, ContextUtilization)
• ContextRelevance
• NoiseSensitivity
• RougeScore
• SemanticSimilarity
• String metrics (ExactMatch, StringPresence, NonLLMStringSimilarity, DistanceMeasure)
• SummaryScore

Documentation Pattern

Each metric documentation follows this structure:

1. Primary Example: Collections-based API (modern, recommended)
   • Uses from ragas.metrics.collections import MetricName
   • Shows llm_factory setup with AsyncOpenAI
   • Direct parameter passing to ascore()
   • Includes note about sync .score() method
2. How It's Calculated: Conceptual explanation (implementation-agnostic)
3. Legacy Section: Original API for backward compatibility
   • Uses SingleTurnSample and .single_turn_ascore()
   • Clearly labeled as legacy pattern

Test Plan

• Build docs locally to verify formatting
• Test code examples to ensure they work
• Verify all metrics from collections are documented
• Check cross-references and links