feat: what is a top layer article draft

[jh3y]

trafficstars

Changes proposed in this pull request:

• Brief explainer article on the browser's top layer

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site/en/blog/what-is-the-top-layer/index.md

• This file passed all of our automated Markdown audits.

[rachelandrew]

@jh3y you are good to merge this when you are ready. I've updated the date to today, but you can change that if you like.