Add documentation for FileCache getRef API method

[dev-docs-github-app-dev]

This PR adds comprehensive documentation for the FileCache getRef method, which provides access to cache entry reference counts. The documentation includes:

• Detailed method signature and parameters
• Return value specifications
• Usage examples with code snippets
• Implementation details including thread-safety considerations
• Information about related cache implementations

The new documentation is located in opensearch/api/file-cache.md and serves to:

1. Improve API discoverability for developers
2. Provide clear usage guidelines and examples
3. Document thread-safety considerations and implementation details
4. Highlight the method's utility for debugging and monitoring cache behavior

This documentation addition will help developers better understand and utilize the FileCache reference counting functionality in their applications.

[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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```