docs: Computed env vars could really use an example

[philBrown]

What problem does this feature solve?

The documentation at https://cli.vuejs.org/guide/mode-and-env.html#using-env-variables-in-client-side-code offers no example for computed env vars.

As can be seen in this StackOverflow question, it is not clear where or how to add a computed env var.

What does the proposed API look like?

It would be great if there was some sample code, eg

process.env.VUE_APP_VERSION = require('./package.json').version

module.exports = {
  // other config, eg configureWebpack
}