Update static-site-generators.md

[aromeoes]

Describe your changes

Next.js guide was missing, even though it was specified in the top of the site

What issue(s) does this address?

Lack of next.js upload example

Does this update depend on any other PRs?

no

Checklist before requesting a review

• Passing the beta version of the Check Markdown links for modified files check. Action results can be viewed here.

Checklist before merging

• Passing all required checks (The beta Check Markdown links for modified files check is not required)

[welcome]

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