tidb-cloud: add storage billing explanations

[lilin90]

What is changed, added or deleted? (Required)

Expand the billing doc with storage model guidance, including standard row-based storage, Infrequent Access row-based storage, and TiFlash-powered columnar storage. The update also adds IA caveats (experimental/region-limited), enablement SQL for TiFlash replicas, and dual-layer encryption details with compliance-focused use cases.

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[gemini-code-assist]

Code Review

This pull request adds new documentation sections to tidb-cloud-billing.md covering Row-based storage, Row-based storage with Infrequent Access (IA), Columnar storage, and Dual-layer encryption. The review feedback focuses on aligning the text with the style guide by adopting the active voice and second-person perspective, removing repetitive wording, fixing a sentence fragment, and using lowercase for common nouns.

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[zhaoshangzi]

仅保留 ： Standard row-based storage stores data in TiKV.