tidb-cloud: add description for Starter agent plan

[lilin90]

What is changed, added or deleted? (Required)

• Added a note for Starter agent plan
• Refined the wording for Starter intro
• Updated branch overview

Note

The URL of the application form is to be finalized. I'll update it after it's ready.

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[gemini-code-assist]

Code Review

This pull request updates the TiDB Cloud documentation to clarify the free quota for the Starter plan and introduces the new Agent Plan for platform builders. The review feedback focuses on improving clarity and conciseness by adhering to style guide principles, such as using active voice and present tense instead of passive voice and future tense.

[lilin90]

/hold

[qiancai]

Rest LGTM

[ti-chi-bot]

[LGTM Timeline notifier]

Timeline:

• 2026-05-12 10:55:03.374530529 +0000 UTC m=+176671.907309848: ☑️ agreed by qiancai.

[ti-chi-bot]

@qiancai: Your lgtm message is repeated, so it is ignored.

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