Add external links to links.json artifact

[reakaleek]

Context

For our Links Validation Service, we need to generate a list of external links (links pointing to external documentation in our system) that will be uploaded to the Link Index.

Changes

Add external links to the links.json

Resulting JSON file:

{
  "origin": {
    "branch": "feature/add-external-links",
    "remote": "elastic/docs-builder-unknown",
    "ref": "03c22a29e6585b72f64a0aa64f087dd5664c939f"
  },
  "url_path_prefix": "",
  "internal_links": [
    "index.md",
    "configure/index.md",
    "configure/page.md",
    "testing/external-links.md",
    "...",
  ],
  "external_links": [
    "kibana://path/to/file.md",
    "kibana://path/to/another/file.md"
  ]
}

[reakaleek]

@Mpdreamz suggested a more performant way of doing this. Instead of making the changes in this PR, I will close this and create a new one.