ADMIN related statements: add more links and fix format

[Oreoxmt]

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)

• Add links for ADMIN related statements
• List the ADMIN related statements in sql-statements/sql-statement-admin.md
• Fix SQL commands style

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• v6.2 (TiDB 6.2 versions)
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• Delete files
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[ti-chi-bot]

[REVIEW NOTIFICATION]

This pull request has been approved by:

• TomShawn

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[TomShawn]

LGTM

[Oreoxmt]

/merge

[ti-chi-bot]

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

Commit hash: ac43a37

[Oreoxmt]

/merge

[ti-chi-bot]

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

Commit hash: d54ec66

[ti-chi-bot]

In response to a cherrypick label: new pull request created: #10911.

[ti-chi-bot]

In response to a cherrypick label: new pull request created: #10912.

[ti-chi-bot]

In response to a cherrypick label: new pull request created: #10913.