Documentation: Cluster Core Concept Documentation

## What would you like to be added?
Enhance the documentation for Karpor Cluster core concept to provide clear and comprehensive information.

Requirements Source: #783

## Why is this needed?
The current documentation lacks detailed explanations of Cluster core concepts, which may confuse users and hinder effective utilization of the feature. This enhancement will:

1. Provide a thorough definition of Clusters in Karpor
2. Detail key attributes and their significance
3. Explain the Cluster lifecycle from creation to deletion
4. Offer usage scenarios and practical examples.

This improved documentation will guide users in understanding and effectively managing Clusters within Karpor.