docs: clarify free trial vs AWS Marketplace subscription

[AllenMok]

What

Clarifies the relationship between the default free trial and AWS Marketplace subscription in tidb-cloud/tidb-cloud-billing.md:

• Adds a note under Payment method: new TiDB Cloud accounts include a free trial by default, so there is no need to subscribe to an AWS Marketplace free trial or link an AWS billing account to access the trial.
• Adds a note under the AWS Marketplace tab: subscribing to a free trial through AWS Marketplace requires creating a new TiDB Cloud account and cannot be linked to an existing account. Users who already have an account and want to pay through AWS should follow the billing account linking steps instead.

🤖 Generated with Claude Code

Summary by CodeRabbit

• Documentation
  • Clarified that new TiDB Cloud accounts automatically include a free trial without requiring an AWS Marketplace subscription or linked AWS billing account.
  • Documented that AWS Marketplace free trials require a new TiDB Cloud account and cannot be linked to an existing account.
  • Added guidance for existing account holders to use the billing account linking process.

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ℹ️ Recent review info

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📥 Commits

Reviewing files that changed from the base of the PR and between 884aa5a and 18f5abd.

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• tidb-cloud/tidb-cloud-billing.md

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📝 Walkthrough

Walkthrough

The billing documentation adds notes about default TiDB Cloud free trials and AWS Marketplace free trial account requirements.

Changes

Billing trial guidance

Layer / File(s)	Summary
Trial and marketplace account notes tidb-cloud/tidb-cloud-billing.md	The documentation states that new accounts receive a default free trial without AWS Marketplace subscription or billing account linking. It also states that AWS Marketplace trials require new TiDB Cloud accounts and cannot link to existing accounts.

Priority: ⬇️ Low

Estimated code review effort: 1 (Trivial) | ~5 minutes

Change: Other

Suggested reviewers: qiancai

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title clearly and concisely describes the main documentation change: clarifying the difference between the default free trial and AWS Marketplace subscriptions.
Description check	✅ Passed	The description clearly explains the documentation changes and their purpose. It omits the required TiDB version selection and related-link information, but the main change description is complete.
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check. Docstring coverage is scoped to functions touched by this diff. Analyzed 0 functions across 0…
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.

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