Disaster recovery solutions documents

[killme2008]

We need DR documents to describe disaster recovery (DR) solutions provided by GreptimeDB(Enterprise):

• Overview @killme2008
• Dual Active-Standby DA @MichaelScofield
• Multiple Replicas Cluster DA @fengjiachun
• Cross zone DA @fengjiachun
• Backup & Restore DA @nicecui

It's a high-priority task.

Let's finish them ASAP. @fengjiachun @MichaelScofield @nicecui