docs: document the exports field in package.json

[njg7194]

Summary

This PR adds documentation for the exports field in the manifest (package.json) documentation.

Changes

Added comprehensive documentation for the exports field, which is the modern and recommended way for packages to define their public API and entry points.

Documentation includes:

• Main field description: Explains the purpose of exports and its relationship to legacy fields like main and module
• Subpath exports: How to map specific subpaths like "./utils" to different files
• Conditional exports: Using conditions like "import", "require", "node", "default" to provide different implementations
• Pattern exports: Using * wildcards to match multiple subpaths
• publishConfig.exports: Added the ability to override exports at publish time

Example

{
  "exports": {
    ".": {
      "import": "./dist/index.mjs",
      "require": "./dist/index.cjs"
    },
    "./utils": "./dist/utils.js"
  }
}

Closes #6909