docs: add skill format contribution guide

[RerankerGuo]

Summary

• add an English skill format and contribution guide
• add the matching Chinese guide
• link the guide from both Manager guide variants

Tests

• git diff --check

Addresses part of #735.

[RerankerGuo]

@shiyiyue1102 Hi! This PR adds the Skill format/contribution guide under docs/ to close one slice of #735 (public roadmap + good-first-contribution guidance). It covers skill locations, directory layout, frontmatter fields, writing rules, and a review checklist, with both English and Chinese versions, and is linked from both Manager guide variants.

translate is green and the branch is MERGEABLE against current main. Docs-only, low-risk. Could you take a look when you have a moment? Happy to adjust wording/structure.