docs: add public-facing product roadmap (zh/en/ja/ko)

[qiongyu1999]

What

Add a public-facing product roadmap page to the docs site, available in all four locales (zh/en/ja/ko).

Why

The team lacked a user-facing roadmap. Users had no visibility into what's shipped, what's being built, and what's coming next. This establishes a clear, maintained signal for users and contributors.

How

• Rewrote roadmap content in benefit-driven language (Linear-style): each item explains what users can now do or no longer have to deal with, not just what was implemented
• Structured around three status tiers: Shipped / Building / Planned
• Added roadmap pages for all four locales: docs/zh/guide/roadmap.md, docs/en/guide/roadmap.md, docs/ja/guide/roadmap.md, docs/ko/guide/roadmap.md
• Wired all four pages into their respective sidebars in docs/.vitepress/config.ts

Affected areas

• docs/.vitepress/config.ts — sidebar entries for en/zh/ja/ko
• docs/*/guide/roadmap.md — new pages (4 files)

Checklist

• Dead links: no references to non-existent pages
• All four locales created and wired into sidebar
• Benefit-driven language throughout (not feature-list style)

[cloudflare-workers-and-pages]

Deploying nexu-docs with    Cloudflare Pages

Latest commit:	e4c0307
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Branch Preview URL:	https://docs-product-roadmap.nexu-docs.pages.dev

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[chatgpt-codex-connector]

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[chatgpt-codex-connector]

Keep Seedance Windows setup requirements consistent

This change points users to the official Windows install guide, but this page still lists 已安装 Windows 内测版 nexu as a prerequisite above, which creates contradictory setup instructions for the same flow. In practice, users on the stable installer can’t tell whether they are supported, and users following the old beta-only assumption may stop unnecessarily. Please update this section so the prerequisite wording matches the new official-install path.

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