Support Markdown admonitions

[2372281891]

click here to see the example

[Havvy]

That isn't Markdown, it's an extension. That said, said extension would be useful for the reference. I wouldn't include it in base mdbook, so how do we extend mdbook for support like this, if at all?

Okay, just read the readme, and I think I'll try my hand at doing this.

[ehuss]

I'm guessing the original was linking to https://docusaurus.io/docs/markdown-features/admonitions, though it is not clear. Features like this are best implemented as a preprocessor.

[tv42]

It seems this can be done with a plugin just fine: https://crates.io/crates/mdbook-admonish