Update link registry listings to include git reference

[Mpdreamz]

https://elastic-docs-link-index.s3.us-east-2.amazonaws.com/link-index.json

 "repositories": {
    "apm-agent-android": {
      "main": {
        "repository": "apm-agent-android",
        "path": "elastic/apm-agent-android/main/links.json",
        "branch": "main",
        "etag": "c976499cd8fb9036e5d4ed0a4bbeeacb"
      }
    },

This will add a ref with the sha of the commit that produced the links.json.

@reakaleek I left "main" as the key here instead of using current and next.

Instead I removed the .First() instances and made all the crosslinks resolvers explicitly look for the key they are after.