📝 Docs: Add a banner/notice to the current site linking to the new site

[JoshuaKGoldberg]

Documentation Request Checklist

• I have read and agree to Mocha's Code of Conduct and Contributing Guidelines
• I have searched for related issues and issues with the faq label, but none matched my issue.
• I want to provide a PR to resolve this

Overview

Nothing on the current https://mochajs.org indicates there's a new website coming. Most website readers probably have no idea it exists. I think it'd be useful to add some indication to the website that a new one is coming.

Proposal: how about a floating banner on top of the page saying something like?

We're working on a website revamp. Preview the new site here: (link)

Additional Info

IMO it would be good to wait until #5303 is resolved. That way we're staying on the mochajs.org domain, not sending to some third-party (netlify) site. But in theory if we really want this doesn't need to be blocked.

#5246 has a description of the rough plan I was thinking of while working on the PR. https://github.com/orgs/mochajs/projects/8/views/1 is a more full milestone with that plan fleshed out.