Document Re-enrollment when agents are moved to another cluster

[michalpristas]

DoD

• Docs covering dependent work is created
• Warning users to create sufficient number of fleet server prior to executing re-enrollment process is present

Dependencies

• Depends on Handle Migrate action in agent elastic-agent#7772
• Depends on Single agent migration - UI elements kibana#217616
• Depends on Bulk migration - UI elements kibana#217619

Resources

Important background content in parent topic

[nimarezainia]

@kilfoyle putting this on your radar.

[nimarezainia]

@karenzone as per our discussion.