Documentation on how to create "Tracking plugins"

We would like to add extra tracking code to our site such as Enhanced Link Attribution ( https://support.google.com/analytics/answer/2558867?hl=en&utm_id=ad ) I see on your documentation that you do this with "Tracking Plugins" but I cannot find documentation on how to create my own or any other way to add extra tracking code to the site.

I've tried registering a new .js file to add my extra code but it is always added to the page before the tracking code even if it is the last file in portal_javascripts.

Start by reading the existing documentation on “Tracking Plugins” and the portal_javascripts behavior described in the issue. Document how to create a tracking plugin or otherwise add extra tracking code, including how script ordering works, and confirm that the guidance addresses Enhanced Link Attribution.

Written by the indexing model from the issue text.