[🐛 in esl-media]: background video should not be a focusable element

[ala-n]

Steps to reproduce

1. Create a background video with the default HTML video provider (e.g. https://esl-ui.com/examples/embedded-media/)
2. Do not provide any a11y markers to make it presentational
3. Navigate through focusable elements on the page

Current behavior:

video is focusable

Expected behaviour:

background video should not be focusable

Tech notes:

Make sure we provide an easy way to declare the role of the video
E.g. use alt or explicit role attribute. Check that a11y note is presented in the documentation