docs: Explain environment variables from `package.json`, `.npmrc` and `npm`

[goastler]

Environment variables explanation was incorrect before.

• [x] fixed capitalisation of headings
• [x] added the env vars populated from package.json
• [x] added the env vars populated from .npmrc
• [x] added the env vars populated from npm

I haven't been able to get any more env vars from the package.json other than the name, version, engines, config and bin. If more env vars are populated, I'll adjust the documentation, but they're the only ones afaict