Expand our visual regression testing to include 'non-default' scenarios

[vanitabarrett]

In #2070 we added Percy visual regression testing to govuk-frontend. It takes screenshots of each component with JavaScript disabled and enabled. There are other contexts which we might want to consider generating screenshots for, that will help us capture issues that we might otherwise not notice or forget to test, such as:

• Compatibility mode
• High contrast (don't think Percy has built-in support for this, but we could explore alternatives/workarounds)

Who needs to work on this

Developers

Who needs to review this

Developers

Done when

• Brainstormed scenarios which we might want to generate screenshots for
• Compared the scenarios we want to implement vs how much of our 100,000 monthly screenshot limit we're currently using
• Changed the Percy test to generate additional screenshots and checked they're appearing in Percy.io as expected
• Updated documentation

[colinrotherham]

Looks like we've covered this one

We reviewed the Percy screenshots to add:

Example pages

• Add “Text alignment” and “Typography” pages to Percy #3332

Additional component examples

• Add Percy screenshots for “inverse” components #3959
• Add Percy screenshots for specific component examples #3962
• Add custom checkboxes and radios examples to Percy #4108