Add GitHub style markdown alerts

[kaste]

Fixes #296

Also adds header anchors for deep linking into the readme.

[kaste]

That seems to work okay, and looks okay too.

@braver for a quick look maybe.

• the links (only in the readme) now have underlines. I think that is actually important but it is on a separate commit so easy to undo.
• in theory, different note types(?) could have different colors; I didn't do that here; I also had no packages at hand where this is used heavily.
• The heading anchors are traditional links but don't have text in it and no aria-* labels; so that's already not so super good; you can tab them but the outline is ugly 🤷

Nothing that can't be improved later as well.