docs: add template-level URL override (#6212)

docs/networking.md

@@ -22,6 +22,18 @@ NAT between workspaces and coder server.
 Users connect to the coder server's dashboard and API through its `ACCESS_URL`
 as well. There must not be a NAT between users and the coder server.
 
+Template admins can overwrite the site-wide access URL at the template level by
+leveraging the `url` argument when [defining the Coder provider](https://registry.terraform.io/providers/coder/coder/latest/docs#url):
+
+```terraform
+provider "coder" {
+  url = "https://coder.namespace.svc.cluster.local"
+}
+```
+
+This is useful when debugging connectivity issues between the workspace agent and
+the Coder server.
+
 ## Web Apps
 
 The coder servers relays dashboard-initiated connections between the user and