docs: add TiDB Cloud CLI (tdc) Preview documentation

[Icemap]

What changed

• Add complete Preview documentation for TiDB Cloud CLI (tdc), including quick start, concepts, command guides, examples, and references.
• Cover TiDB Cloud Starter, TiDB Cloud Filesystem, FS Git, Journal, Vault, configuration, security, regions, installation, and updates.
• Add problem-driven agent scenarios and explain background Git workspace hydration.
• Register every page in the AI documentation navigation.

Why

tdc needs task-oriented product documentation that gets new users to a working database or Filesystem quickly while still documenting the complete command surface and the tdc/Drive9 boundary.

User impact

Users and coding agents can follow supported release-install workflows, understand credential and platform boundaries, and apply tdc to concrete database, shared-workspace, Git, journal, and secret-delegation scenarios.

Validation

• PingCAP markdownlint
• Remark lint and PingCAP anchor validation
• Internal and external link checks
• Current tdc command and flag coverage checks
• English/Chinese command-block parity checks

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[Icemap]

Chinese Translation PR(from this PR): pingcap/docs-cn#21821