Add File Cache documentation with reference count retrieval details

[dev-docs-github-app-dev]

This PR adds comprehensive documentation for the File Cache component, specifically focusing on the recently added reference count retrieval functionality.

Key additions include:

• Created new file-cache.md documentation explaining the File Cache component and its functionality

• Detailed documentation of the new getRef method including:

  • Method signature and parameters
  • Usage examples with code snippets
  • Implementation details
  • Thread safety considerations
  • Use cases and benefits

• Added implementation details covering:

  • FileCache class
  • LRUCache class
  • SegmentedCache integration
  • Thread safety mechanisms

The documentation will help developers understand:

• How to retrieve and use cache entry reference counts
• Best practices for cache monitoring and debugging
• Thread-safety considerations when working with the File Cache

These additions improve the overall documentation coverage of OpenSearch's caching system and provide valuable information for developers working with the File Cache component.

[dev-docs-github-app-dev]

This pull request was created by AI Agent. Please review the changes and provide feedback. Context used:

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