Add documentation for Remote File Cache functionality

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

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+# Remote File Cache
+
+The Remote File Cache is a feature in OpenSearch that manages caching of files for remote store operations. This document describes the functionality and usage of the `FileCache` class, which is part of the remote store implementation.
+
+## Overview
+
+The `FileCache` class is introduced to solve the problem of local disk space limitations when working with remote stores. It maintains a node-level view of index files with priorities, caching only the index files needed by queries. The file with the lowest priority (Least Recently Used) in the FileCache is replaced first.
+
+## Key Features
+
+- Prioritized caching of index files
+- Least Recently Used (LRU) eviction policy
+- Thread-safe operations
+
+## Main Interfaces
+
+The `FileCache` class provides two main interfaces:
+
+1. `put`: Adds a new file index input to the cache.
+2. `get`: Retrieves a file from the cache and updates its priority.
+
+## Usage
+
+### Adding a File to the Cache
+
+To add a file to the cache:
+
+```java
+fileCache.put(filePath, indexInput);
+```
+
+This operation adds the file at the cache head, giving it the highest priority.
+
+### Retrieving a File from the Cache
+
+To get a file from the cache:
+
+```java
+CachedIndexInput indexInput = fileCache.get(filePath);
+```
+
+This operation does not add the file to the cache but promotes its priority by making it the most recently used.
+
+## Eviction
+
+When the file cache reaches its capacity, it starts evictions. The eviction process removes file items from the cache tail and triggers a callback to clean up the file from disk. The cleanup process also includes closing the file's descriptor.
+
+## Thread Safety
+
+The `FileCache` class is designed to be thread-safe, allowing concurrent access from multiple threads.
+
+## Additional Methods
+
+- `capacity()`: Returns the maximum capacity of the cache.
+- `size()`: Returns the current number of items in the cache.
+- `clear()`: Removes all items from the cache.
+
+## Implementation Details
+
+The `FileCache` uses a `SegmentedCache` internally, which provides better concurrency by sharding the cache into multiple segments. This reduces contention when multiple threads are accessing the cache simultaneously.
+
+## Conclusion
+
+The Remote File Cache provides an efficient way to manage index files for remote store operations in OpenSearch. By prioritizing frequently accessed files and evicting least recently used ones, it helps optimize storage usage and improve query performance when working with remote stores.