feat(site): enable vitepress-plugin-llms for LLM-friendly docs (eggjs#5764)

Add vitepress-plugin-llms plugin to generate llms.txt and llms-full.txt
files during site build, making documentation accessible for LLMs.

Also fix syntax highlighting typo in unittest.md (bas -> bash).

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## Summary by CodeRabbit

* **New Features**
* Integrated a documentation plugin that adds markdown copy/download
buttons and other authoring enhancements to the docs site, and
registered it with the site build.
* **Documentation**
* Fixed a code block language spec for correct syntax highlighting in
the unittest example.
* **Chores**
  * Added the plugin dependency to the site manifest.

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settings.</sub>
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Co-authored-by: Claude Opus 4.5 <noreply@anthropic.com>

Author: fengmk2