Feature request: Vitepress-style callout markdown support

Hello! I’m a new user to Nextra but it is hands-down the best React-powered docs solution I’ve run across. I love how quickly it is to get up-and-running with almost no config, and I love how great everything looks, feels, and works ❤️.

Feature Request

Nextra has Callouts, but requires importing the component manually. Vitepress, instead, has a more automatic built-in syntax:

::: info
This is an info box.
:::

::: tip
This is a tip.
:::

::: warning
This is a warning.
:::

::: danger
This is a dangerous warning.
:::

::: details
This is a details block.
:::

This is not only easier to compose; it also works in regular ol’ .md files for more flexibility / portability. And not that the Nextra project needs to be compatible with Vitepress or anything, but in general the more parity this project has with others, the easier it is for people to migrate to it.

If this syntax seems inline with the project, I’d love to add a PR hooking up this syntax with the Callout component.