Add documentation for amazon_web_services.eks_kms_arn config option

docs/docs/explanations/advanced-provider-configuration.md

@@ -98,6 +98,49 @@ amazon_web_services:
   permissions_boundary: arn:aws:iam::01234567890:policy/<permissions-boundary-policy-name>
 ```
 
+### EKS KMS ARN (Optional)
+
+AWS Key Management Service (KMS) keys can provide envelope encryption of Kubernetes secrets stored in
+Amazon Elastic Kubernetes Service (EKS). Kubernetes secrets store sensitive information (e.g. passwords,
+credentials, TLS keys, etc.). Kubernetes stores all secret object data within etcd and all etcd volumes
+used by Amazon EKS are encrypted at the disk-level using AWS-managed encryption keys. The benefit of
+specifying a user-managed KMS key for EKS is adding a level of envelope encryption to apply a security
+best practice of [defense-in-depth strategy](https://aws.amazon.com/blogs/containers/using-eks-encryption-provider-support-for-defense-in-depth/) by encrypting the Kubernetes secrets store with a KMS key
+that the user manages.
+
+Nebari supports setting an existing KMS key while deploying Nebari to implement encryption of secrets
+created in Nebari's EKS cluster. The KMS key must be a **Symmetric** key set to **encrypt and decrypt** data.
+
+:::warning
+Enabling EKS cluster secrets encryption, by setting `amazon_web_services.eks_kms_arn`, is an
+_irreversible_ action and re-deploying Nebari to try to remove a previously set `eks_kms_arn` will fail.
+On the other hand, if you try to change the KMS key in use for cluster encryption, by re-deploying Nebari
+after setting a _different_ key ARN, the re-deploy should succeed but the KMS key used for encryption will
+not actually change in the cluster config and the original key will remain set. The integrity of a faulty
+deployment can be restored, following a failed re-deploy attempt to remove a previously set KMS key, by
+simply re-deploying Nebari while ensuring `eks_kms_arn` is set to the original KMS key ARN.
+:::
+
+:::danger
+If the KMS key used for envelope encryption of secrets is ever deleted, then there is no way to recover
+the EKS cluster.
+:::
+
+:::note
+After enabling cluster encryption on your cluster, you must encrypt all existing secrets with the
+new key by running the following command:
+`kubectl get secrets --all-namespaces -o json | kubectl annotate --overwrite -f - kms-encryption-timestamp="time value"`
+Consult [Encrypt K8s secrets with AWS KMS on existing clusters](https://docs.aws.amazon.com/eks/latest/userguide/enable-kms.html) for more information.
+:::
+
+Here is an example of how you would set KMS key ARN in `nebari-config.yaml`.
+
+```yaml
+amazon_web_services:
+  # the arn for the AWS Key Management Service key
+  eks_kms_arn: 'arn:aws:kms:us-west-2:01234567890:key/<aws-kms-key-id>'
+```
+
 ### Launch Templates (Optional)
 
 Nebari supports configuring launch templates for your node groups, enabling you to customize settings like the AMI ID and pre-bootstrap commands. This is particularly useful if you need to use a custom AMI or perform specific actions before the node joins the cluster.