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.

No files or tests are named. Start by locating Docsify’s Markdown parsing entry point and compare its behavior with the Markdown Guide footnote syntax shown in the issue. The work is done when references such as [^1] and definitions render correctly, with the placement of footnotes decided and covered by tests.

Written by the indexing model from the issue text.