Document serving static files using Plack::Middleware::Static

Documents an example of serving static files using Plack::Middleware::Static in the Dancer2::Cookbook. Also adds a note at [Sessions and logging in](https://metacpan.org/pod/distribution/Dancer2/lib/Dancer2/Cookbook.pod#Sessions-and-logging-in) example about it preventing static files to be served when the user is not logged in.