Clarify setup for outbound click tracking

[nztomas]

Sorry it's not very clear from your documentation if I need to add the additional .js file or if everything is already included in the module so when I switch on the option, the additional JS is included automatically.

"Outbound Link Click Tracking" is an enhanced measurement and not included in our default script. This is because we want to keep the default script as simple and lightweight as possible. The [additional enhanced measurements](https://plausible.io/docs/script-extensions) you can choose to add depending on your needs.

Detailed Description

Context

Possible Implementation