📝 Docs: Add rel=sponsored to backer/sponsor links

[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

As pointed out by @voxpelli: external sponsor links on mochajs.org just have rel="noopener". Per https://developers.google.com/search/docs/crawling-indexing/qualify-outbound-links, sponsored should also be added to their rel (preferred over nofollow).

Additional Info

Co-authored-by @voxpelli