Add contextual example with secondary and warning buttons

[36degrees]

What

Add one of more 'full-page' examples that show the secondary and warning buttons in a realistic context.

Why

We're updating the focus style as part of making the Design System meet WCAG 2.1 – specifically with regard to WCAG 2.1 1.4.11 Non-Text Contrast (#1137).

As part of this we are getting our full-page examples audited. We want to ensure that these newly introduced variants are included in the audit

Done when…

• Realistic example defined
• Example(s) added to the review app which show the secondary and warning buttons in context