Add short notes to explain about customizing domain and customizing domain template (#4319)

* Add short notes to explain about customizing domain and customizing domain template.

The following two sections have the same title and instruct how to customize domain:

https://knative.dev/docs/serving/using-a-custom-domain/
https://knative.dev/docs/developer/serving/services/custom-domains/

It should have different title and a note the difference.

So this patch changes to:
- Add a short explanation and the link for each section.
- Change the title for domain template to "Customizing domain template".

Fix https://github.com/knative/docs/issues/4256

* Update docs/developer/serving/services/custom-domains.md

Co-authored-by: Samia Nneji <snneji@vmware.com>

* Update docs/serving/using-a-custom-domain.md

Co-authored-by: Samia Nneji <snneji@vmware.com>

* Change title

* Update docs/developer/serving/services/custom-domains.md

Co-authored-by: Samia Nneji <snneji@vmware.com>

* Update docs/developer/serving/services/custom-domains.md

Co-authored-by: Samia Nneji <snneji@vmware.com>

* Update nav.yaml

Co-authored-by: Samia Nneji <snneji@vmware.com>

config/nav.yml

@@ -150,7 +150,7 @@ nav:
         - Configuring HTTPS connections: serving/using-a-tls-cert.md
         - Enabling auto-TLS certs: serving/using-auto-tls.md
         - Configuring the ingress gateway: serving/setting-up-custom-ingress-gateway.md
-        - Setting up a custom domain: serving/using-a-custom-domain.md
+        - Changing the default domain: serving/using-a-custom-domain.md
         - Converting a Kubernetes Deployment to a Knative Service: serving/convert-deployment-to-knative-service.md
       - Code samples:
         - Overview: serving/samples/README.md

docs/developer/serving/services/custom-domains.md

@@ -13,6 +13,11 @@ serve a Knative Service at this domain.
     If you create a domain mapping to map to a [private Knative Service](private-services.md),
     the private Knative Service is accessible from public internet with the custom domain of the domain mapping.
 
+!!! tip
+    This topic instructs how to customize the domain of each service, regardless of the default domain.
+    If you want to customize the domain template to assign the default domain name,
+    see [Changing the default domain](../../../serving/using-a-custom-domain.md).
+
 ## Prerequisites
 
 - You must have access to a Kubernetes cluster, with Knative Serving and an Ingress implementation installed. For more information, see the [Installation documentation](../../../admin/install/README.md).

docs/serving/using-a-custom-domain.md

@@ -1,4 +1,4 @@
-# Setting up a custom domain
+# Changing the default domain
 
 By default, Knative Serving routes use `example.com` as the default domain. The
 fully qualified domain name for a route by default is
@@ -6,6 +6,11 @@ fully qualified domain name for a route by default is
 
 To change the {default-domain} value there are a few steps involved:
 
+!!! tip
+    Customizing a domain template affects your cluster globally.
+    If you want to customize the domain of each service, use `DomainMapping` instead.
+    For more information, see [Configuring custom domains](../developer/serving/services/custom-domains.md).
+
 ## Edit using kubectl
 
 1. Edit the domain configuration config-map to replace `example.com` with your