Add File Cache API documentation

[dev-docs-github-app-dev]

This PR adds comprehensive documentation for the File Cache API, including:

• Detailed API reference for retrieving file cache reference counts
• Path parameters and example request/response formats
• Implementation details of the file cache system
• Usage considerations and best practices
• Links to related documentation

The new file-cache.md documentation helps developers understand how to:

• Monitor and debug file cache usage through reference count queries
• Properly integrate with the file cache system
• Understand thread-safety considerations and performance implications

Note: Documentation includes a warning about the API's internal nature and potential for future changes.

[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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```