Add support for remarkable admonitions

[lkngtn]

https://github.com/favoloso/remarkable-admonitions#with-docusaurus

[yeqbfgxjiq]

Not sure I follow what's going on here. Remarkable is the thing that created that "security vulnerability" that GitHub keeps bugging us about. Is this to fix that, or for something else?

[lkngtn]

I think this allows for more call out types in remarkable markdown render (what Docusaurus uses). Which would be useful when writing documentation.