Update the node-gyp alpine docs section to account for Node 18 in Alpine 3.20 python issue

[LaurentGoderre]

Technically, this is only needed for Node 18 on Alpine 3.20 but since it's part of the virtual group and gets deleted, this is the simplest without overcomplicating the docs.

• Documentation
• Version change (Update, remove or add more Node.js versions)
• Variant change (Update, remove or add more variants, or versions of variants)
• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Other (none of the above)

Checklist

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have read the CONTRIBUTING.md document.
• All new and existing tests passed.