docs(button): explain what a FAB is

[donroyco]

Bug, feature request, or proposal:

Proposal

What is the expected behavior?

Explain what a FAB is in the docs.

What is the current behavior?

No explanation in the docs.

What are the steps to reproduce?

See https://material.angular.io/components/button/overview and https://material.angular.io/components/sidenav/overview where FABs is mentioned, but no (small) explanation of what it is.

What is the use-case or motivation for changing an existing behavior?

It's good to be people one step forward to add this small information, in case they don't have looked into the Material Design guidelines.

Is there anything else we should know?

I created a small PR to resolve this issue: #5790

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