Add explanation to dual-layer encryption

[sunxiaoguang]

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[gemini-code-assist]

Summary of Changes

Hello @sunxiaoguang, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request significantly enhances the TiDB Cloud security documentation by introducing a comprehensive explanation of dual-layer encryption. The update aims to provide users with a clearer understanding of how their data is protected at rest, detailing the mechanisms and applicability across different TiDB Cloud cluster types. This improves the clarity and completeness of the security concepts documentation.

Highlights

• Dual-layer Encryption Explanation: A new section has been added to the security-concepts.md documentation, providing a detailed explanation of dual-layer encryption.
• Cluster-specific Encryption Details: The new content clarifies the definition of dual-layer encryption, how data is encrypted at rest, and its default or mandatory status for Starter, Essential, and Dedicated TiDB Cloud clusters.

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[gemini-code-assist]

Code Review

This pull request adds a helpful explanation of dual-layer encryption to the security concepts documentation. My review provides a few suggestions to improve the clarity, conciseness, and flow of the new content, ensuring it aligns with the repository's style guide. The proposed changes aim to make the explanation of the two encryption layers more explicit and improve overall readability.

[gemini-code-assist]

For better clarity and adherence to the style guide's preference for active voice (line 43), consider rephrasing this sentence. Using 'encrypted at rest' instead of 'encrypted-at-rest' would also improve consistency with other parts of the document (e.g., line 230).

Suggested change

	- All persisted data is encrypted-at-rest using the tool of the cloud provider that your cluster is running in.
	- The cloud provider where your cluster is running encrypts all persisted data at rest using its native tools.

References

1. Passive voice overuse. "The cluster is started by TiUP" → "TiUP starts the cluster" ^(link)

[gemini-code-assist]

This sentence could be clearer about this being the second layer of encryption, building upon the cloud provider's encryption mentioned previously. The suggested phrasing makes this relationship explicit.

Suggested change

	- With dual-layer encryption enabled, data is automatically encrypted at rest using CMEK or escrow keys.
	- With dual-layer encryption enabled, TiDB Cloud adds a second layer of security by automatically encrypting data at rest using either CMEK or escrow keys.