Improve docs from user feedback: clarify fork support for preview API

[mintlify]

Summary

Clarify that preview deployments cannot be built from fork branches, on the Trigger preview deployment API page and in a new help center article.

Changes

• Added a "Branch requirements" section to /api/preview/trigger stating the branch must exist in the connected repository, fork branches are not supported, and linking to the fork workaround.
• Added help center article /help-center/preview-deployment-not-created-for-fork-branch covering the missing-preview symptom for fork PRs and API calls, linking to the canonical Fork pull requests section.
• Registered the new article in the Help center group in docs.json.

Feedback basis

• A reader marked the Trigger preview deployment API page as not helpful because it did not say whether the endpoint works with fork branches. The fork limitation was documented only on the Preview deployments page, not on the API page.
• Dashboard feedback

Other feedback in the window was positive with no corrective signal, so no other pages were changed.

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Note

Low Risk
Documentation-only changes with no product or API behavior impact.

Overview
Documents that preview deployments cannot use fork branches, addressing feedback that the Trigger preview deployment API page did not state this clearly.

Adds a Branch requirements section on api/preview/trigger (and es, fr, zh locales) explaining that branch must exist on the connected repo, fork branches are unsupported because the GitHub App is only on the main repo, with a link to the fork PR workaround.

Introduces help center article preview-deployment-not-created-for-fork-branch in English and localized copies, covering missing preview links on fork PRs, failed API calls with fork branches, and the maintainer push-to-upstream workflow. Registers the page in Help center navigation via docs.json, es.json, fr.json, and zh.json.

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