Please support markdown footnotes [^1] [^note]

## Feature request

#### What problem does this feature solve?

To cite the [Markdown Guide](https://www.markdownguide.org/extended-syntax/#footnotes):

> Footnotes allow you to add notes and references without cluttering the body of the document.

For instance, in docsified software manuals this would help with structuring the information given better: useful background information can be given and clearly linked with the main text, yet without making the main text too full of details.

#### What does the proposed API look like?

According to the [Markdown Guide](https://www.markdownguide.org/extended-syntax/#footnotes):

```md
A footnote[^1]

[^1] This is a footnote.
```

#### How should this be implemented in your opinion?

I'm not sure I understand this question in the context of this particular feature request.

One thing I notice it that docify authors might want to have control over the placement of footnotes: either at the end of a "page" or, alternatively, before the next same-level heading.

#### Are you willing to work on this yourself?

Unfortunately, I completely lack the necessary Javascript and markdown parsing experience to code this.

未指定文件或测试。首先定位 Docsify 的 Markdown 解析入口点，并将其行为与 issue 中展示的 Markdown Guide 脚注语法进行比较。当 [^1] 之类的引用和定义能够正确渲染，并且脚注的位置得到确定且有测试覆盖时，工作即告完成。

由索引模型根据 Issue 内容生成。