Add File Cache API documentation

[dev-docs-github-app-dev]

This PR adds documentation for the File Cache API, providing a comprehensive guide for internal and developer reference. The documentation includes:

• Introduction to the File Cache API and its purpose
• Detailed description of the Get Reference Count endpoint
• Request/response formats with examples
• Implementation details with code snippets
• Usage notes and context for internal operations

The documentation is structured to help developers understand:

• How to interact with the file cache API
• The role of reference counting in cache management
• Integration points with OpenSearch's internal operations

This new documentation fills a gap in our technical documentation coverage and provides valuable reference material for developers working with OpenSearch's caching system.

[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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```