Add essential migration doc

[xuanyu66]

First-time contributors' checklist

• [x] I've signed the Contributor License Agreement, which is required for the repository owners to accept my contribution.

What is changed, added or deleted? (Required)

Which TiDB version(s) do your changes apply to? (Required)

Tips for choosing the affected version(s):

By default, CHOOSE MASTER ONLY so your changes will be applied to the next TiDB major or minor releases. If your PR involves a product feature behavior change or a compatibility change, CHOOSE THE AFFECTED RELEASE BRANCH(ES) AND MASTER.

For details, see tips for choosing the affected versions.

• [ ] master (the latest development version)
• [ ] v9.0 (TiDB 9.0 versions)
• [ ] v8.5 (TiDB 8.5 versions)
• [ ] v8.1 (TiDB 8.1 versions)
• [ ] v7.5 (TiDB 7.5 versions)
• [ ] v7.1 (TiDB 7.1 versions)
• [ ] v6.5 (TiDB 6.5 versions)
• [ ] v6.1 (TiDB 6.1 versions)
• [ ] v5.4 (TiDB 5.4 versions)

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Do your changes match any of the following descriptions?

• [ ] Delete files
• [ ] Change aliases
• [ ] Need modification after applied to another branch
• [ ] Might cause conflicts after applied to another branch