Improve the focus style guidance to include linked images/ videos

[NickColley]

What

We should improve the focus style guidance to include links that wrap block level content like images and videos.

Why

It's not obvious what focus style to give to images that have links around them and videos

Who needs to know about this

Tim, Mark?

Further detail

Example of video focus state

from https://design-system.service.gov.uk/patterns/addresses/

from: https://companieshouse.blog.gov.uk/2019/10/24/launching-our-streaming-api-service-for-company-data/

Done when

• We have documented how to do style a link around and image or video etc