Add docs for Red #system directive

[smotti]

Right now there's no documentation present for the #system directive introduced with this commit: red/red@c9cfa3f

It's only mentioned in an example for the #call directive in the red/system spec: https://static.red-lang.org/red-system-specs.html#section-16.8

Having it added to the directives of the red docs would help in making people aware that it's possible to call red/system code from red using this directive. Otherwise it's kind of hidden and might only be found by accident or a long search.

Also not sure if i'm the right person to make a pull request for that as, I'm still learning and getting familiar with red and its environment. But I guess the commit message provides enough information (at least for me) about what it is and how to use it. So maybe only one or two examples are missing for a proper doc entry.

Thank you very much!