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/marketing/kustomization.yaml

@@ -0,0 +1,6 @@
+apiVersion: kustomize.config.k8s.io/v1beta1
+kind: Kustomization
+resources:
+  - externalsecret.yaml
+  - deployment.yaml
+  - service.yaml