Add refactored Callout component, replace Call-out [fix #787]

[craigcook]

Description

This refactors the Callout, but also changes the name so it effectively removes and replaces the previous component.

The revised Callout:

1. Removes integrated branding options. We can use Logo/Wordmark components for branding.
2. Treats Compact Callout as a layout variant rather than as a separate component.

This also means a Callout can replace a default centered Hero, and we can finally fully remove the Hero component.

However, since this renames/replaces a component it's a breaking change and will need a major release. We could keep the old name but I prefer it without a dash. But moreover I wanted to make the compact version a variant and not a whole component, so that would deprecate Compact Call-out anyway.

So... if we really care about gradual deprecation and keeping old components for a while to let people transition, we shouldn't change the name. But I want to change the name, and keeping the dashed and undashed names is even messier than just doing it.

• I have documented this change in the design system.
• I have recorded this change in CHANGELOG.md.

Issue

#787

Testing

https://demo2--mozilla-protocol.netlify.app/components/detail/callout--default
http://localhost:3000/components/detail/callout--default

[nathan-barrett]

rwc+ Thats a great looking callout. Just some minor documentation questions