Add documentation for Remote File Cache functionality

[dev-docs-github-app-dev]

This PR adds comprehensive documentation for the Remote File Cache feature, which is a crucial component of OpenSearch's remote store operations.

Key additions include:

• Overview of the Remote File Cache functionality and purpose
• Detailed explanation of key features including prioritized caching and LRU eviction
• Usage examples for adding and retrieving files from the cache
• Technical details about thread safety and implementation using SegmentedCache
• Information about eviction policies and cache management

The new documentation will help developers understand:

• How the Remote File Cache manages local disk space limitations
• The prioritization mechanism for caching index files
• Thread-safe operations for concurrent access
• Eviction policies and cleanup processes

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

This documentation enhances the OpenSearch documentation by providing clear guidance on an important feature for managing remote store operations efficiently.

[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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