docs: update guest pull document

After moving guest pull abilities to CDH, the document of guest pull
should be updated due to new workflow.

Also, replace the diagram of PNG into a mermaid one for better
maintaince.

Signed-off-by: Xynnn007 <xynnn@linux.alibaba.com>

docs/design/kata-guest-image-management-design.md

@@ -49,13 +49,58 @@ Pull the container image directly from the guest VM using `nydus snapshotter` ba
 #### Architecture
 
 The following diagram provides an overview of the architecture for pulling image in the guest with key components. 
-![guest-image-management-architecture](arch-images/guest-image-management-architecture.png)
+```mermaid
+flowchart LR
+    Kubelet[kubelet]--> |1\. Pull image request & metadata|Containerd
+    Containerd-->|2\. Pull image metadata| E
+    Containerd-->Snapshotter[Nydus Snapshotter]
+    Snapshotter-->|3\. Pack image info| Containerd
+    Containerd-->Runtime[Kata Runtime]
+    Runtime-->Hypervisor
+    Hypervisor-->TEE
+    Runtime-->|4\. Pass image info to VM| Agent
+    CDH1-->|6\. Pull image with image info|E[Container Images Registry]
+    subgraph TEE [Virtual Machine]
+      Images[Container Images]-->|7\. Prepare container rootfs|H[Container]
+
+      subgraph CDH [Confidential Data Hub]
+        CDH1[Image Mgmt]
+      end
+
+      CDH-->Images
+      Agent[Kata Agent]-->|5\. Call image pull RPC|CDH
+    end
+```
 
 #### Sequence diagrams
 
 The following sequence diagram depicted below offers a detailed overview of the messages/calls exchanged to pull an unencrypted unsigned image from an unauthenticated container registry. This involves the kata-runtime, kata-agent, and the guest-components’ image-rs to use the guest pull mechanism.
 
-![guest-image-management-details](arch-images/guest-image-management-details.png)
+```mermaid
+sequenceDiagram
+    par Hosts Side
+        Containerd/Kubelet->>runtime.kata_agent: createContainer(ctx,sandbox,c)
+        runtime.kata_agent->>runtime.fs_share_linux: ShareRootFilesystem(ctx,c)
+        runtime.fs_share_linux->>runtime.kata_agent: handleVirtualVolumeStorageObject(c,...,KataVolumeType)
+        runtime.kata_agent->>runtime.kata_agent: handleImageGuestPullBlockVolume(c,virtVolume,vol)
+        runtime.kata_agent->>runtime.fs_share_linux: ret:storage
+        runtime.fs_share_linux->>runtime.kata_agent: ret:sharedFile
+    and Guest Side
+        runtime.kata_agent->>agent.rpc: CreateContainerRequest(cid,...,storages,...,oci,...)
+        agent.rpc->>agent.storage: add_storage(storages...)
+        agent.storage->>agent.storage: StorageHandler.handler(driver)
+        agent.storage->>agent.storage.StorageHandler.ImagePullHandler: create_device(storage)
+        agent.storage.StorageHandler.ImagePullHandler->>agent.confidential_data_hub: pull_image(img,cid,img_metadata)
+        agent.confidential_data_hub->>Confidential Data Hub: pull_image(img,bundle_path)
+        Confidential Data Hub->>agent.confidential_data_hub: ret
+        agent.confidential_data_hub->>agent.storage.StorageHandler.ImagePullHandler: ret: bundle_path
+        agent.storage.StorageHandler.ImagePullHandler->>agent.storage: ret: device
+        agent.storage->>agent.rpc: ret: mount_list
+    and Return
+        agent.rpc->>runtime.kata_agent: ret: ok
+        runtime.kata_agent->>Containerd/Kubelet: ret: ok
+    end
+```
 
 First and foremost, the guest pull code path is only activated when `nydus snapshotter` requires the handling of a volume which type is `image_guest_pull`, as can be seen on the message below:
 ```json
@@ -108,10 +153,10 @@ Below is an example of storage information packaged in the message sent to the k
 ```
 Next, the kata-agent's RPC module will handle the create container request which, among other things, involves adding storages to the sandbox. The storage module contains implementations of `StorageHandler` interface for various storage types, being the `ImagePullHandler` in charge of handling the storage object for the container image (the storage manager instantiates the handler based on the value of the "driver").
 
-`ImagePullHandler` delegates the image pulling operation to the `ImageService.pull_image()` that is going to create the image's bundle directory on the guest filesystem and, in turn, class the image-rs to in fact fetch and uncompress the image's bundle. 
+`ImagePullHandler` delegates the image pulling operation to the `confidential_data_hub.pull_image()` that is going to create the image's bundle directory on the guest filesystem and, in turn, the `ImagePullService` of Confidential Data Hub to fetch, uncompress and mount the image's rootfs. 
 
 > **Notes:**
-> In this flow, `ImageService.pull_image()` parses the image metadata, looking for either the `io.kubernetes.cri.container-type: sandbox` or `io.kubernetes.cri-o.ContainerType: sandbox` (CRI-IO case) annotation, then it never calls the `image-rs.pull_image()` because the pause image is expected to already be inside the guest's filesystem, so instead `ImageService.unpack_pause_image()` is called.
+> In this flow, `confidential_data_hub.pull_image()` parses the image metadata, looking for either the `io.kubernetes.cri.container-type: sandbox` or `io.kubernetes.cri-o.ContainerType: sandbox` (CRI-IO case) annotation, then it never calls the `pull_image()` RPC of Confidential Data Hub because the pause image is expected to already be inside the guest's filesystem, so instead `confidential_data_hub.unpack_pause_image()` is called.
 
 ## Using guest image pull with `nerdctl`
 
@@ -121,6 +166,6 @@ nerdctl run --runtime io.containerd.kata.v2 --snapshotter nydus --label io.kuber
 ```
 
 References:
-[1] [[RFC] Image management proposal for hosting sharing and peer pods](https://github.com/confidential-containers/confidential-containers/issues/137)
-[2] https://github.com/containerd/containerd/blob/main/docs/content-flow.md
-
+1. [[RFC] Image management proposal for hosting sharing and peer pods](https://github.com/confidential-containers/confidential-containers/issues/137)
+2. https://github.com/containerd/containerd/blob/main/docs/content-flow.md
+3. [Move guest pull ability to a configurable component](https://github.com/kata-containers/kata-containers/issues/9266)