Add File Cache API documentation

[dev-docs-github-app-dev]

This PR adds comprehensive documentation for the File Cache feature in OpenSearch. The documentation includes:

• Introduction to the File Cache feature and its purpose
• Detailed coverage of the getRef method implementation across different cache classes
• Method syntax, parameters, and return value specifications
• Implementation details for FileCache, LRUCache, and SegmentedCache classes
• Usage examples and code snippets
• Important considerations and thread-safety notes

This documentation will help developers understand how to work with the File Cache system and manage reference counting in OpenSearch's caching mechanism. The new documentation provides clear examples and implementation details that were previously undocumented.

Location of new documentation:

• opensearch/api/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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```