docs: add uni-app and uni-app x SDK documentation

[Bloomingg]

Summary

• add a frozen, public-safe Private SDK documentation contract for unix-openim-sdk 0.2.0-rc.3
• publish 167 manually reviewed pages in both Chinese and English for uni-app / uni-app x
• cover all 162 operations, 81 subscriptions, 2 event controls, 237 types, and 109 constants with deterministic ownership
• document commercial capabilities, field-level extensions, HarmonyOS unsupported APIs/events, native file paths, lifecycle, and subscription cleanup
• register uni-app in navigation, search, sitemap, localized content, and SDK overview
• fail closed on contract drift, retired APIs, invalid imports, Wasm-style examples, and incomplete review evidence

Verification

• pnpm content:status
• pnpm structure:report
• pnpm content:sync
• pnpm audit --prod
• pnpm check
• pnpm build

All checks pass. The production dependency audit reports no known vulnerabilities.

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