Add detailed documentation about member cluster leave process

[Copilot]

This PR enhances the documentation to provide a comprehensive explanation of what happens when a member cluster leaves the fleet. The updated documentation in the "Leaving the Fleet" section now includes:

1. A step-by-step breakdown of the leave process:

   • How the process is initiated by deleting the MemberCluster CR
   • The coordination between hub and member cluster controllers
   • The state transitions during the leave process

2. Details about resource cleanup:

   • How resources propagated via Fleet are removed from the member cluster
   • The role of ApplyWorkReconciler in removing finalizers and enabling garbage collection
   • How AppliedWork objects are deleted to clean up resources

3. Status reporting and completion:

   • How the member agent reports successful or failed leave operations
   • The final cleanup steps performed on the hub cluster
   • What happens to resource placements targeting the cluster

4. Connection to other documentation:

   • A link to the how-to guide for uninstalling the member agent after the leave process

These changes should help users better understand the expected behavior when a cluster leaves a fleet and what happens under the hood during this process.

Fixes #1133.

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