`External Message Support`: Improve example

[jubnzv]

We have the following example:

{
  "options": {
    "external": true
  }
}

I would write it this way:

{
  "projects": [
    {
      "name": "sample",
      ...
      "options": {
        "external": true
      }
    }
  ]

In order to additionally highlight that's a part of the project configuration. Otherwise we can have questions: https://t.me/tactlang/41482.

[novusnota]

A duplicate of #384 (in some sense, since I was planning to re-write it and fix all the issues), but thanks nonetheless!