Update explanation.md with new security items

- Add "Ceph architecture" section
- Link to new security explanation items
- Move "Security" section up

Author: anantone

charmed-ceph/explanation.md

@@ -1,5 +1,11 @@
 This section provides explanatory and conceptual guides as well as a technical background on topics. It is meant to help you expand your knowledge of the Ceph ecosystem.
 
+## Ceph architecture
+
+An overview of the general Ceph architecture with some of the components that can be deployed.
+
+* [General Ceph architecture](https://ubuntu.com/ceph/docs/ceph-architecture)
+
 ## Storage types
 
 Ceph provides object, block, and file based storage on commodity hardware.
@@ -15,14 +21,20 @@ Pools constitute the basic building blocks of a Ceph cluster. They allow for hig
 * [Replicated pools](/t/18805)
 * [Erasure-coded pools](/t/18807)
 
-## RADOS Gateway multi-site replication
+## Security
 
-Charmed Ceph supports multi-site replication with the Ceph RADOS Gateway. This provides even more resilience to your object storage by replicating it to geographically separated Ceph clusters. This is of particular importance for active backup and disaster recovery scenarios where a site is compromised and access to data would be otherwise lost.
+Learn about security topics applicable to Charmed Ceph.
 
- * [RADOS Gateway multi-site replication](/t/35992)
+* [Attack surface](https://ubuntu.com/ceph/docs/attack-surface)
+* [Access controls](https://ubuntu.com/ceph/docs/access-controls)
+* [Secrets](https://ubuntu.com/ceph/docs/secrets)
+* [Encryption](https://ubuntu.com/ceph/docs/encryption)
+* [Best practices for Secure deployment](https://ubuntu.com/ceph/docs/secure-deployment)
+* [Best practices for Secure operation](https://ubuntu.com/ceph/docs/secure-operation)
+* [Cryptography](https://ubuntu.com/ceph/docs/cryptographic-technologies-in-charmed-ceph)
 
-## Security
+## RADOS Gateway multi-site replication
 
-Learn about security topics applicable to Charmed Ceph.
+Charmed Ceph supports multi-site replication with the Ceph RADOS Gateway. This provides even more resilience to your object storage by replicating it to geographically separated Ceph clusters. This is of particular importance for active backup and disaster recovery scenarios where a site is compromised and access to data would be otherwise lost.
 
-* [Full disk encryption](/t/68576)
+ * [RADOS Gateway multi-site replication](/t/35992)