Add docs for visual comparison expect API

[JosefJezek]

Could you add support for visual testing?

see on this solutions

• https://codecept.io/visual/#using-resemble-helper
• https://theheadless.dev/posts/basics-taking-screenshots/#use-in-visual-regression-testing

[arjunattam]

Thanks @JosefJezek! We have added support for visual comparisons. I'll reframe this to track adding that to our docs.