Add `rel=sponsored` to sponsor links

[ChristianMurphy]

Initial checklist

• [x] I read the support docs
• [x] I read the contributing guide
• [x] I agree to follow the code of conduct
• [x] I searched issues and couldn’t find anything (or linked relevant results below)
• [x] If applicable, I’ve added docs and tests

Description of changes

Matching annotations to the ones used on https://unifiedjs.com/

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Name	Status	Preview	Comments	Updated (UTC)
mdx	✅ Ready (Inspect)	Visit Preview	💬 Add feedback	Feb 12, 2024 7:58pm

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Comparison is base (908ff45) 100.00% compared to head (5eaf608) 100.00%. Report is 7 commits behind head on main.

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Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #2439 +/- ## ========================================= Coverage 100.00% 100.00% ========================================= Files 23 23 Lines 2693 2694 +1 Branches 2 2 ========================================= + Hits 2693 2694 +1 ```

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[wooorm]

This is copy/pasted from the readmes when everything is changed. That’s why it’s nice to have it similar to the rest, minimal changes.

I am not so worried about spam here: these are top doners who I am in contact with. Not random iffy betting things or so.

[wooorm]

Thanks!