Fix broken link in shard-merge-best-practices.md and addd a missing section (#900)

[ti-chi-bot]

This is an automated cherry-pick of #900

First-time contributors' checklist

• I've signed Contributor License Agreement that's required for repo owners to accept my contribution.

What is changed, added, or deleted? (Required)

1. Fix broken link in shard-merge-best-practices.md reported by Broken Link Detected #898.
2. Fix a section that is missing in translation from zh: add detailed checks and steps for shard-merge scenario #698 to update toc.md, shard-merge.md, add incremental data migration document #748

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

Since December 23, 2021, the TiDB DM documentation in the master branch has been merged to TiDB documentation by #8042 and #7317. To create a PR for TiDB DM versions later than v5.3, either go to TiDB English documentation repository or Chinese documentation repository.

• v5.3 (TiDB DM 5.3 versions)
• v2.0 (TiDB DM 2.0 versions)
• v1.0 (TiDB DM 1.0 versions)

What is the related PR or file link(s)?

• This PR is translated from:
• Other reference link(s):

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

[ti-chi-bot]

[REVIEW NOTIFICATION]

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• shichun-0415

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[shichun-0415]

/merge

[ti-chi-bot]

This pull request has been accepted and is ready to merge.

Commit hash: 9a389f8

[shichun-0415]

/merge