Nested/Recursive includes

### Problem or Desire

For our new docs at work, we want to convert LaTeX to Markdown. The problem is that it has quite a lot of nested documents with `\include`, which Docsify can only go one level deep.

### Proposal

Docsify should resolve the included markdown files recursively. With protection against looping embeds.

### Additional Context

There is no need to make it a setting to preserve backwards compatibility.
First time

该 issue 未指明文件、测试或入口点。首先定位 Docsify 对 markdown \include 指令的处理方式，以及现有的 embed 解析测试；然后验证递归包含，包括防止 embed 循环的保护机制，同时保持当前行为，不添加设置。

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