Support GitHub-style blockquote admonitions

[wlach]

Describe the feature you'd like to request

GitHub supports admonitions via a slightly different syntax than MyST:

https://github.com/orgs/community/discussions/16925

e.g.

> [!NOTE]  
> Highlights information that users should take into account, even when skimming.

It would be great if you could write admonitions in either style and have them render, as sometimes documentation is read on both GitHub and as part of a Sphinx docset.

Describe the solution you'd like

Support this feature (probably via an opt-in syntax extension).