feat: add callout support

[isobelmcrae]

adds support for github-style callouts

• only the warning is styled properly, i haven't thought about colours for other types at the moment - if someone chooses to use them, they can implement the styling if needed before i get to it
• also converts the current warning-emoji style disclaimers to use the new callouts

an example:

[Zopolis4]

this is good but maybe the capitalisation and the callout support should be at the very least separate commits if not separate PRs

[l1mey112]

Looks fine to me