Provide procedures to scale up TiKV clusters using local disks to another node pool about docs-tidb-operator

[DanielZhangQD]

@cofyc @weekface @lichunzhu WDYT?

[cofyc]

gke, or all cloud providers (aws, gke, aliyun)?

[cofyc]

I think it's useful but has a low priority compared to other issues in current docs.

[DanielZhangQD]

The user reports this issue is using GKE, but I think we have to cover EKS at least.
And @zhexuany would like to submit for PR for this, then we can review.

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[DanielZhangQD]

Also in some cases, users want to migrate to nodes with better disks, e.g. nodes with HDD to nodes with SSD, we can provide a similar procedure as https://docs.pingcap.com/tidb-in-kubernetes/dev/migrate-tidb-to-kubernetes#step-2-create-a-tidb-cluster-in-kubernetes, just create another tc, set the spec.pdAddress to the pd address of the original tc, then they will become one whole tidb cluster, and we can scale in the components in the original tc manually.

[DanielZhangQD]

Also, even if with EBS, users may want to shrink the EBS volume size.