feat: Add user guide for compaction offload (#158)

Co-authored-by: Jiacai Liu <dev@liujiacai.net>

content/en/docs/user-guide/cluster_deployment/with_meta.md

@@ -187,9 +187,28 @@ timeout = "5s"
 server_addrs = ['http://{HoraeMetaAddr}:2379']
 ```
 
+### Compaction Offload
+
+Compaction offload is also supported. To enable compaction offload, the corresponding compaction mode with node picker and endpoint should be configured.
+
+- `node_picker`: There are two types of node picker -- `Local` and `Remote`(WIP).
+  - When the `Local` is setted, the local compaction task would be offloaded to the specific remote compaction server, which decided by `endpoint`.
+- `endpoint`: The endpoint, in the form `addr:port`, indicating the _grpc port_ of the remote compaction server.
+
+Here is an example for it:
+
+```toml
+[analytic.compaction_mode]
+compaction_mode = "Offload"
+node_picker = "Local"
+endpoint = "{RemoteCompactionServerAddr}:{RemoteCompactionServerGrpcPort}"
+```
+
+A Compaction Server, responsible for executing the compaction task, is also needed. Currently `horaedb-server` will act as this role, in the future we can move it to an independent service.
+
 ### Complete Config of HoraeDB
 
-With all the parts of the configurations mentioned above, a runnable complete config for HoraeDB can be made. In order to make the HoraeDB cluster runnable, we can decide to adopt RocksDB-based WAL and local-disk-based Object Storage:
+With all the parts of the configurations mentioned above, a runnable complete config for HoraeDB can be made. In order to make the HoraeDB cluster runnable, we can decide to adopt RocksDB-based WAL and local-disk-based Object Storage without compaction offload:
 
 ```toml
 [server]