docs: document gypfile field in package.json

[MaxBlack-dev]

Description

This PR adds documentation for the \gypfile\ field in package.json, which was previously undocumented.

Changes

• Added a new section documenting the \gypfile\ field
• Explained that npm automatically builds packages with binding.gyp using node-gyp
• Documented that setting \gypfile: false\ prevents automatic building
• Included example showing how to disable the automatic build

Fixes

Closes #6898

Context

The \gypfile\ field is currently respected by
pm rebuild\ (as documented in the rebuild command docs), but the field itself was not documented in the package.json specification. This change ensures users understand how to control node-gyp build behavior for native addons.

Type of Change

• Documentation update