[Website] Add stylized "Warning" callouts

[myronmarston]

Suggest an improvement

There are some warnings included in our docs but they aren't highlighted in any way to make them standout. We should improve that. Two examples:

elasticgraph/config/site/src/query-api/filtering/list.md

Lines 21 to 26 in b44c963

	{% comment %}TODO: figure out a way to highlight this section as a warning.{% endcomment %}
	One thing to bear in mind: this query is selecting which _artists_ to return,
	not which _albums_ to return. You might expect that the returned `nodes.albums` would
	all be platinum-selling 90s albums, but that's not how the filtering API works. Only artists
	that had a platinum-selling 90s album will be returned, and for each returned artists, all
	their albums will be returned--even ones that sold poorly or were released outside the 1990s.

elasticgraph/config/site/src/query-api/filtering/conjunctions.md

Line 54 in b44c963

{% comment %}TODO: figure out a way to highlight this section as a warning.{% endcomment %}

URLs of impacted pages

https://block.github.io/elasticgraph/query-api/filtering/list/
https://block.github.io/elasticgraph/query-api/filtering/conjunctions/

Screenshots

Here's how those pages look now: