docs: document searchable opt-in for hidden tabs and groups

[mintlify]

Summary

Documents the new searchable: true option that you can set alongside hidden: true on a tab or group. With it, the tab or group stays out of the rendered navigation, but its descendant pages remain indexed for site search, the sitemap, the AI assistant, llms.txt, skill.md, and search engines. This is the documented path for using hidden tabs as a per-product left-nav workaround without silently breaking search and SEO.

Changes

• Added a "Keep hidden tabs and groups searchable" section to organize/hidden-pages.mdx with examples for both tabs and groups and a description of how searchable interacts with hidden as the navigation tree is walked.
• Mirrored the new section into the es, fr, and zh translations of organize/hidden-pages.mdx.

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Note

Low Risk
Low risk documentation-only change that clarifies configuration behavior without modifying runtime logic.

Overview
Adds a new section to organize/hidden-pages.mdx explaining how to keep pages under hidden tabs/groups discoverable by setting searchable: true, including examples and the precedence rules between hidden and searchable.

Mirrors the same documentation update in the Spanish (es), French (fr), and Chinese (zh) translations.

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