Alert dialog guidelines

Problem

Now that we have the wonderful Dialog documentation out, we might want to expand the documentation around dialog when used for alerts, warnings, or destructive purposes.

As @edokoa pointed out here, this sort of dialog doesn't follow a clear set of patterns, therefore we end up with different variations often used for similar use cases.

Proposal

Add a new Dialog usage section (similar to the Button usage), or a subsection inside the Messaging guidelines. This documentation could cover:

• Alert/flash banner usage (including best practices of danger and warning variants, positioning, tone and voice, etc)
• Heading content, so we don't vary from "Are you sure...?" to "Doing this..."
• Guidelines to display output or consequences on important actions (where and how to them, tone and voice, etc)
• Consistency around buttons (when to require or not a confirmation input, using danger or secondary buttons, etc)

Slack thread reference (internal)