refactor: rename k8s-manifests directory to manifests

- Rename all `k8s-manifests` subdirectories and files to `manifests`
- Update all file paths in `argocd-init.yml` to reflect the new directory name

🤖

manifests/cert-manager/selfsigned-issuer.yaml

@@ -0,0 +1,10 @@
+---
+apiVersion: cert-manager.io/v1
+kind: ClusterIssuer
+metadata:
+  name: selfsigned
+  namespace: cert-manager
+  annotations:
+    argocd.argoproj.io/sync-options: SkipDryRunOnMissingResource=true
+spec:
+  selfSigned: {}