Improving docs callouts

[corywatilo]

Right now we use <blockquote> for callouts in docs/handbook. For basic callouts, they work fine. But they don't work well for:

• Alerts or warnings where we want to use an icon
• When we have alerts/warnings that we want to make multi-line, as we then have to switch them to raw html instead of Markdown

It would be nice to have a component for more complex callouts.

Here are designs for callouts containing:

1. An icon (optional)
2. Text
3. Title (optional)
4. CTA (optional)

(These use our existing Tailwind colors, some of which could be improved, but we should stick with using them for now.)