html: callout color should respect brand

[gordonwoodhull]

We've discussed this internally but I can't find an issue or discussion.

For Typst we have a mapping from bootstrap callout names to brand yml theme colors.

quarto-cli/src/resources/filters/customnodes/callout.lua

Lines 235 to 241 in bc45896

	local callout_theme_color_map = {
	note = "primary",
	warning = "warning",
	important = "danger",
	tip = "success",
	caution = nil -- ?
	}

We should generalize this and use it for HTML.