docs: Inform users that independ dashboard is optional

[nolouch]

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[Oreoxmt]

Suggested change

	### 访问独立部署的 TiDB Dashboard (可选)
	### 访问独立部署的 TiDB Dashboard（可选）

[Oreoxmt]

Suggested change

	### 部署独立的 TiDB Dashboard (可选)
	### 部署独立的 TiDB Dashboard（可选）

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PR needs rebase.

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[Oreoxmt]

Please update the base branch for this PR from master to release-1.x, as release-1.x is now the active development branch for TiDB Operator v1.x documentation.