docs(i18n): add a Chinese contributor guide for the CLA flow

[bestony]

Problem

CONTRIBUTING.md now defines the repository's ICLA and CCLA paths, but the
repository has no Chinese contributor guide. Chinese-speaking contributors
should be able to understand the CLA requirement without creating a second,
independently maintained contribution process.

This is separate from #893, which aligns entry-point links in
README_zh-CN.md but does not translate the contributor guide or its CLA flow.

Proposed Direction

Add a Chinese companion for CONTRIBUTING.md that clearly identifies the
English file as canonical and mirrors the full public contribution journey,
including the individual and corporate CLA paths.

Acceptance Criteria

• Add CONTRIBUTING.zh-CN.md and link to canonical CONTRIBUTING.md near
  the top.
• Translate the full contributor journey, including local setup, required
  checks, PR conventions, ICLA signing, CCLA verification, issue filing,
  and security reporting.
• Preserve the exact ICLA signing phrase in English so the automation can
  recognize it.
• State that the English guide and the pinned English agreement documents
  control if translations differ.
• Link the Chinese guide from README_zh-CN.md without defining a separate
  Chinese-only workflow.
• Verify every relative and external link.

Context

• CLA implementation: feat: add CLA signing workflow #2023
• Chinese README alignment: docs(i18n): align the Chinese README contribution path with the English README #893