📝 Docs: add Skill pages to documentation sidebars

[yaodong-shen]

Summary

• add Skill Management to the English and Chinese User Guide sidebars
• add the backend Skills landing page and architecture overview to both Backend Development sidebars
• keep the navigation structure symmetric across locales

Problem

The bilingual Skill pages already exist and render, but the VitePress sidebars provide no route to them. Users must know the direct URLs to find the documentation.

Testing

• pnpm run docs:build
• VitePress 1.6.4 build completed successfully with dead-link validation

SPEC

No SPEC update is needed. This is a single-file documentation-navigation fix and does not change APIs, database schemas, runtime contracts, cross-module behavior, or product behavior.

Closes #3501

[yaodong-shen]

SonarCloud has passed. Docker Build Doc Check and CodeQL Advanced are waiting for maintainer approval and created no jobs for this external-fork PR. The VitePress documentation build passes locally. Could a maintainer approve the workflow runs when convenient?