Add Remote File Cache documentation for getRef method

[dev-docs-github-app-dev]

This PR adds comprehensive documentation for the Remote File Cache module, specifically focusing on the getRef method implementation. The documentation includes:

• Overview of the Remote File Cache functionality
• Detailed explanation of the getRef method in FileCache class
• Description of the RefCountedCache interface updates
• Implementation details for LRUCache and SegmentedCache
• Usage examples and code snippets
• Thread-safety considerations

This documentation will help developers better understand how to monitor and manage cached files in OpenSearch deployments using remote storage, particularly when tracking reference counts of cached items for debugging and optimization purposes.

Located in: opensearch/reference/index-modules/store/remote-file-cache.md

[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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```