[Docs] Fix the illustration of plugin workflow

[haochengxia]

• Fix the error in the mermaid figure
• Add extra description when the user wants to implement the existing eviction algorithm via the plugin system
• Fix links in TOC

[gemini-code-assist]

Summary of Changes

Hello @haochengxia, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request focuses on improving the documentation for the plugin workflow. It rectifies an incorrect flow in an existing Mermaid diagram and introduces a new section to clarify challenges and provide guidance for users attempting to re-implement cache eviction algorithms using the plugin system, particularly concerning how state updates should be handled when the find function is not directly exposed to plugins.

Highlights

• Documentation Correction: Corrected an erroneous flow in the Mermaid diagram within doc/quickstart_plugin.md to accurately represent the cache_eviction_hook returning to the "Cache full?" check.
• Plugin Implementation Guidance: Added a new section detailing potential implementation issues when re-implementing eviction algorithms using the plugin system, emphasizing the need to handle state-update logic within hook functions due to the find function not being exposed. This includes a new Mermaid diagram for clarity.

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[gemini-code-assist]

Code Review

This pull request improves the plugin documentation by correcting a workflow diagram and adding a helpful section on implementation challenges when using the plugin system. The changes are clear and valuable for developers using plugins. I have one suggestion to improve the clarity of the new diagram to make it more consistent with the accompanying text.