cloud: add a tip about how to request a feature in private preview

[qiancai]

What is changed, added or deleted? (Required)

• Add a tip about how to request a feature in private preview
• Decrease the size of ❌

Which TiDB version(s) do your changes apply to? (Required)

Tips for choosing the affected version(s):

By default, CHOOSE MASTER ONLY so your changes will be applied to the next TiDB major or minor releases. If your PR involves a product feature behavior change or a compatibility change, CHOOSE THE AFFECTED RELEASE BRANCH(ES) AND MASTER.

For details, see tips for choosing the affected versions.

• [ ] master (the latest development version)
• [ ] v9.0 (TiDB 9.0 versions)
• [ ] v8.5 (TiDB 8.5 versions)
• [ ] v8.1 (TiDB 8.1 versions)
• [ ] v7.5 (TiDB 7.5 versions)
• [ ] v7.1 (TiDB 7.1 versions)
• [ ] v6.5 (TiDB 6.5 versions)
• [ ] v6.1 (TiDB 6.1 versions)
• [ ] v5.4 (TiDB 5.4 versions)

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• This PR is translated from:
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Do your changes match any of the following descriptions?

• [ ] Delete files
• [ ] Change aliases
• [ ] Need modification after applied to another branch
• [ ] Might cause conflicts after applied to another branch

[gemini-code-assist[bot]]

Summary of Changes

Hello @qiancai, I'm Gemini Code Assist[^1]! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request enhances the tidb-cloud/features.md documentation by providing clear instructions for users interested in requesting private preview features. It also includes a minor visual adjustment to the feature support table, making the '❌' symbols consistently smaller for improved readability.

Highlights

• Private Preview Feature Request Tip: Added a new tip section to the tidb-cloud/features.md document, guiding users on how to request features that are currently in private preview through the TiDB Cloud console.
• Styling Adjustment for '❌' Symbol: Modified the CSS styling for the '❌' symbol within the feature support table in tidb-cloud/features.md to decrease its font size to 14px for better visual presentation.

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[ti-chi-bot[bot]]

[LGTM Timeline notifier]

Timeline:

• 2025-12-18 09:57:37.073705128 +0000 UTC m=+1726201.887482710: :ballot_box_with_check: agreed by lilin90.

[qiancai]

/approve

[ti-chi-bot[bot]]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: qiancai

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