Add dev/stg/prod environments with per-env values, configmaps and secrets

- Move env-specific config into environments/<env>/{values,configmap,secret}.yaml
  for both nginx-chart and mysql-chart (dev, stg, prod)
- Make templates env-agnostic: resource names come from values
  (configMap.name, secret.name)
- nginx deployment mounts env ConfigMap and loads env Secret via envFrom
- mysql statefulset reads DB config from ConfigMap, passwords from Secret
- Remove mysql-chart/templates/secret.yaml (secrets are now per-env manifests)
- Add README with layout and deploy commands

mysql-chart/environments/dev/secret.yaml

@@ -0,0 +1,12 @@
+# MySQL credentials for the DEV environment (referenced by the StatefulSet).
+# Replace with real values; for production use a sealed/encrypted secret instead
+# of committing plaintext.
+apiVersion: v1
+kind: Secret
+metadata:
+  name: mysql-secret-dev
+  namespace: dev
+type: Opaque
+stringData:
+  MYSQL_PASSWORD: demo123
+  MYSQL_ROOT_PASSWORD: root123