[Bug]: missing documentation on generated `package_json.bzl`

### What happened?

The rules_js repo does not currently have any documentation on how to use the macros generated in `package_json.bzl` by `npm_import`. This came up in the context of https://github.com/aspect-build/rules_js/pull/2846.

### Version

Development (host) and target OS/architectures: all

Output of `bazel --version`: all versions

Version of the Aspect rules, or other relevant rules from your
`WORKSPACE` or `MODULE.bazel` file: n/a

Language(s) and/or frameworks involved: n/a

### How to reproduce

```shell

```

### Any other information?

_No response_

Start by reading the npm_import context and pull request #2846, then inspect the generated package_json.bzl mentioned in the issue. Document how to use its generated macros, with enough guidance for users to apply them from the existing rules_js documentation.

Written by the indexing model from the issue text.