feat(istio): enhance Istio tutorial document

ℹ️ We're making the Istio tutorial more user-friendly! 💡 We've added extra
context and instructions related to how external-dns can gather information
from the Kubernetes service of the Istio Ingress Gateway. Also, we've guided
users on setting targets manually with the appropriate annotation. 😎

Changes include:
- Illustration on how to get targets to extracted DNS names using external-dns
- Guidance to the source service documentation for more understanding 📚
- Steps to manually set targets using the 'external-dns.alpha.kubernetes.io/target'
annotation on the Istio Ingress Gateway resource or the Istio VirtualService.

Author: KarstenSiemer

docs/tutorials/istio.md

@@ -246,6 +246,11 @@ spec:
 EOF
 ```
 
+To get the targets to the extracted DNS names, external-dns is able to gather information from the kubernetes service of the Istio Ingress Gateway.
+Please take a look at the [source service documentation](../sources/service.md) for more information on this.
+
+It is also possible to set the targets manually by using the `external-dns.alpha.kubernetes.io/target` annotation on the Istio Ingress Gateway resource or the Istio VirtualService.
+
 #### Access the sample service using `curl`
 ```bash
 $ curl -I http://httpbin.example.com/status/200