docs(roadmap): fix dead Contributing link (404) in EN+ZH roadmap

[r266-tech]

Problem

Both roadmap pages invite contributions, but the Contributing link 404s:

• docs/en/about/03-roadmap.md L115 — See [Contributing](contributing.md) for guidelines.
• docs/zh/about/03-roadmap.md L115 — 请参阅 [贡献指南](contributing.md)。

Each link resolves relative to docs/{en,zh}/about/ → docs/{en,zh}/about/contributing.md, which does not exist anywhere in the repo tree. The contribution guidelines actually live at the repo root (CONTRIBUTING.md / CONTRIBUTING_CN.md). A contributor who follows the roadmap's "We welcome contributions … See Contributing" call-to-action hits a dead page.

Fix

Retarget each link to the canonical root file via its GitHub blob URL — the same convention README.md already uses for root-level repo resources (e.g. the LICENSE badge → .../blob/main/LICENSE):

• EN → https://github.com/volcengine/OpenViking/blob/main/CONTRIBUTING.md
• ZH → https://github.com/volcengine/OpenViking/blob/main/CONTRIBUTING_CN.md

Pure docs, 2 lines, no content change beyond the link target. Restores the contribution funnel on a stable nav page.

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