Mark External Links - any Ideas?

I am newbie to grav ( it really rocks ! ) and not a web developer, but have decades of programming experience.

On my Grav-Homepage, I want to mark external links as such, for legal reasons.
And I want it automatically to happen, to avoid forgetting it.
It should be like adding a little Icon and style to a link, to clearly show that it is an external link,
but ONLY if it is external, i.e. if it has “http://” or “https://” in it.

Maybe there is a way to "hooK’ into the markdown engine/parser, or a method to “filter” markdown content dynamically ( to add an icon before https:// ).

Or maybe it can be done solely with css, or twig …

Any ideas ?

I Found