support GitHub-style asides

[QuietMisdreavus]

Radar: rdar://107569909

GitHub supports "Note" and "Warning" callouts with a syntax similar to Swift-Markdown:

> **Note**
> This is a note.

> **Warning**
> This is a warning.

Rendered output

Note
This is a note.

Warning
This is a warning.

Swift-Markdown (and by extension Swift-DocC) should be able to support these varieties of callouts as native asides. This may require work in Swift-DocC as well, depending on how and where asides are converted.

[grynspan]

Per https://github.com/orgs/community/discussions/16925, looks like there are a couple of different syntaxen for this.

> [!WARNING]
> Alert! Alert!

> **Warning**
> Callooh! Callay!

They render identically:

Warning

Oh me! Oh my!