[REST] create-an-attestation should mention that private repositories need to be owned by enterprises

[jsoref]

Code of Conduct

• I have read and agree to the GitHub Docs project's Code of Conduct

What article on docs.github.com is affected?

https://docs.github.com/en/rest/repos/repos?apiVersion=2022-11-28#create-an-attestation

What part(s) of the article would you like to see updated?

Restructure this sentence:

The authenticated user must have write permission to the repository and, if using a fine-grained access token, the attestations:write permission is required.

Something like this might work:

• The authenticated user must have write permission to the repository.
• If the repository is private, it must be owned by an enterprise.
• If using a fine-grained access token, the attestations:write permission is required.

Additional information

• Document the requirement for an enterprise plan for private repositories. actions/attest#193
• https://github.blog/news-insights/product-news/introducing-artifact-attestations-now-in-public-beta/#sigstore-in-public-and-in-private

[jsoref]

This documentation is reached from errors like this:
https://github.com/check-spelling-sandbox/gh-foorbar/actions/runs/12626392159/attempts/1

Error: Failed to persist attestation: Feature not available for the check-spelling-sandbox organization. To enable this feature, please upgrade the billing plan, or make this repository public. - https://docs.github.com/rest/repos/repos#create-an-attestation

[nguyenalex836]

@jsoref Thank you for raising this issue and providing all of that context! I'll get this triaged for review ✨ Our team will provide feedback regarding the best next steps for this issue - thanks for your patience! 💛

[github-actions]

Thanks for opening an issue! We've triaged this issue for technical review by a subject matter expert 👀