docs: align skill import package guidance

[Ill-donut]

Summary

• align the library Skill import package wording with the Agent Build guide
• clarify that the package needs a SKILL.md file and that the default 50 MB limit is configurable
• update English, Chinese, and Japanese together

Checks

• English formatting check
• Chinese and Japanese formatting check
• git diff --check

[RiskeyL]

LGTM