Add banner to next.fakerjs.dev

[ST-DDT]

Clear and concise description of the problem

Currently is it hard to spot whether you are on a production branch or our development branch.

Alternatively

Suggested solution

Show a banner with a hint that the users is currently viewing the development branch.

It might be useful to always show the banner if the hostname is not fakerjs.dev.

The banner should contain a link to the production website (https://fakerjs.dev).

Alternative

No response

Additional context

This originally came up here: #1438 (comment)

[Shinigami92]

@brc-dd told me we can use layout-top https://vitepress.vuejs.org/guide/theme-introduction#layout-slots for that 👍
I can tackle this, if no one else is interested in this. But first I would like to get some other of my PRs in a review-able state or even merged, so I don't have too many branches open from my site.