📝 docs: add plugin docs for the new third stage

[arvinxx]

💻 变更类型 | Change Type

• ✨ feat
• 🐛 fix
• ♻️ refactor
• 💄 style
• 🔨 chore
• 📝 docs

🔀 变更说明 | Description of Change

补充：

• 插件触发机制 [Question] 插件的触发方式是什么？ #25
• manifest 基本概念介绍
• 插件类型介绍(default、markdown、standalone)
• 插件开发基础概念
• Plugin Gateway 介绍
• OpenAPI 集成介绍

close #37

📝 补充信息 | Additional Information

refs: lobehub/lobe-chat#411

[lobehubbot]

👍 @arvinxx


Thank you for raising your pull request and contributing to our Community
Please make sure you have followed our contributing guidelines. We will review it as soon as possible.
If you encounter any problems, please feel free to connect with us.
非常感谢您提出拉取请求并为我们的社区做出贡献，请确保您已经遵循了我们的贡献指南，我们会尽快审查它。
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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

❗ No coverage uploaded for pull request base (master@23c9e08). Click here to learn what that means.

Additional details and impacted files

@@            Coverage Diff            @@
##             master      #39   +/-   ##
=========================================
  Coverage          ?   98.81%           
=========================================
  Files             ?       21           
  Lines             ?     1182           
  Branches          ?      106           
=========================================
  Hits              ?     1168           
  Misses            ?       14           
  Partials          ?        0           

☔ View full report in Codecov by Sentry.
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[lobehubbot]

❤️ Great PR @arvinxx ❤️


The growth of project is inseparable from user feedback and contribution, thanks for your contribution!
项目的成长离不开用户反馈和贡献，感谢您的贡献!

[lobehubbot]

🎉 This PR is included in version 1.32.1 🎉

The release is available on:

• npm package (@latest dist-tag)
• GitHub release

Your semantic-release bot 📦🚀