Add liquid templates for alerts, tabs, and expanders

These are not super clean just because of the way jekyll/liquid/github works but I think they should get us what we want without resorting to HTML. Though tbh I think the HTML is sometimes simpler (says the person who knows HTML).

Anyway you can find examples for how to use everything in the code. Note: spacing is VERY important in markdown and should generally be collapsed if you don't have any reason to include it. Otherwise you get extra <p> tags.

@gtback @bworrell @ikiril01 @bschmoker can you guys review this to make sure it's semi-sane? And to come up with less-terrible ideas for how to do the same thing?