cloud: update cloud intro

[lilin90]

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Mention Premium an Lake in the beginning descriptions on What is TiDB Cloud | TiDB Docs, to improve completeness

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Reviewing files that changed from the base of the PR and between 8c21ce6 and bcd82cb.

📒 Files selected for processing (2)

• tidb-cloud/select-cluster-tier.md
• tidb-cloud/tidb-cloud-intro.md

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Timeline:

• 2026-09-15 08:20:18.328843217 +0000 UTC m=+106864.266500811: ☑️ agreed by qiancai.

[lilin90]

/approve

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This pull-request has been approved by: lilin90

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