Button: add guidance for not having danger icon only buttons

[laurenmrice]

Acceptance criteria

We do not support danger icon only buttons in code. A common question has come up from our users why we do not support this and we need to be more clear about the reasoning behind this in our website documentation.

Tasks

• Add guidance to the Button Usage tab of why we do not support danger icon only buttons. This section might be a good place to explain this.
• Add some images (like Don/Dont images) to support this reasoning.
• Some reasons why we don't support this:
  • We believe that danger can be a critical action and should be applied to a button a higher emphasis that also includes a visual label, like a primary, tertiary, or ghost button.
  • There has sometimes been confusion on what color to use for the different states in a UI. Sometimes we have seen product teams using the default ghost button color tokens for the enabled state, while the danger color tokens appear on hover, which feels disjointed and is inaccurate.