Support Alerts

[benbarbour]

Feature Description

Hugo now supports Alerts, via blockquote render hooks. These are compatible with both Github and Obsidian markdown, and I think are easier to type and easier to use than "callout" shortcodes. They also look prettier in markdown. Hextra should have a default render hook for them.

Hugo's alerts also support folding, which steps on the toes of Hextra's Details shortcode, but Hextra should probably support it with Alerts anyway.