chore: use Node.js 20 for docs sites

[mdjermanovic]

Prerequisites checklist

• I have read the contributing guidelines.

What is the purpose of this pull request? (put an "X" next to an item)

[ ] Documentation update
[ ] Bug fix (template)
[ ] New rule (template)
[ ] Changes an existing rule (template)
[ ] Add autofix to a rule
[ ] Add a CLI option
[ ] Add something to the core
[x] Other, please explain:

I've updated configurations on Netlify to use Node.js 20 when building docs and org sites.

This updates docs' package.engines to reflect this environment so that we can use dependencies that don't support older versions of Node.js.

Also updates Node.js version for the Browser test.

What changes did you make? (Give an overview)

Updated engines in docs/package.json, and updated the CI workflow per the above points.

Is there anything you'd like reviewers to focus on?

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