Clean up API Documentation CSS and bring a more consistent experience across devices [API v2 QA]

[anikbrazeau]

Description

Look at the design of the API documentation with an eye for visual and interaction design.
Consider changes to colours, typography and other design elements.

Tablet / Edge:

Mac / Chrome:

[anikbrazeau]

cc @samsadasivan

[anikbrazeau]

https://cds-snc.github.io/forms-api/

[anikbrazeau]

#355