Expand Automatic Rebalancing

[jseldess]

• The intro should probably summarize how and why we do this, and cross-reference https://cockroachlabs.com/docs/stable/architecture/replication-layer.html#membership-changes-rebalance-repair.
• The tutorial should cover load-based rebalancing as well.

Follow-up to #3921.

[jseldess]

Consolidated this tutorial with https://www.cockroachlabs.com/docs/dev/demo-fault-tolerance-and-recovery.html.