Add a setting to set attributes (at least target="_blank") for doc links

[2.x 29b110f] Issue #3493869: Added documentation to config fields privacyUri and imprintUri how to achieve target="_blank" and rel="noopener nofollow" to links.

Based on the changes in the JS library cookiesjsr I documented in the form fields how link attribution behaves based on the given URI.
See: https://github.com/jfeltkamp/cookiesjsr/issues/23

@jfeltkamp good enough IMHO, thank you :)