deploy neuron marketing site to neurontechnologies.ai

Add marketing/ manifests (Deployment, Service, ExternalSecret) for the
Next.js marketing site in neuron-prod namespace, an Argo CD app pointing
to the new path, and update the prod ingress to serve / from neuron-marketing
instead of neuron-rest.

servers/legion/k8s/neuron-technologies/prod/ingress.yaml

@@ -70,9 +70,9 @@ spec:
             pathType: Prefix
             backend:
               service:
-                name: neuron-rest
+                name: neuron-marketing
                 port:
-                  number: 8081
+                  number: 3000
     - host: www.neurontechnologies.ai
       http:
         paths:
@@ -80,6 +80,6 @@ spec:
             pathType: Prefix
             backend:
               service:
-                name: neuron-rest
+                name: neuron-marketing
                 port:
-                  number: 8081
+                  number: 3000