Docusaurus v3 deprecates :::caution admonition

[andrew-polk]

See https://docusaurus.io/blog/preparing-your-site-for-docusaurus-v3#admonition-warning.

With v3, you are supposed to use either :::danger or :::warning. :::caution will still show the yellow (caution) admonition, but it is deprecated.
But in v2, :::warning produces the red (danger) admonition.

So, we will probably need to introduce a flag indicating if you are targeting v2 or v3. (Of course, we will likely find there are other important factors depending on which you are targeting when we start testing on v3.

This is not urgent since v3 will continue to support :::caution.