docs: translate the Chinese doc of scale.md into English #1793 (#1945)

Co-authored-by: LU MIAN <lumianph@gmail.com>

docs/en/maintain/index.rst

@@ -9,4 +9,5 @@ Maintenance
     backup
     monitoring
     cli
-    faq
+    faq
+    scale

docs/en/maintain/scale.md

@@ -0,0 +1,74 @@
+# Scale-Out and Scale-In
+
+## Scale-Out
+
+With the development of business, dynamic scale-out is required if the current cluster topology cannot meet the requirements. Scale-out is to migrate some partitions from existing tablet nodes to new tablet nodes.
+
+### Step 1. Starting a new tablet node
+
+After startup, you need to check whether the new node has joined the cluster. If the `showtablet` command is executed and the new node endpoint is listed, it means that it has joined the cluster
+
+```bash
+$ ./bin/openmldb --zk_cluster=172.27.128.31:8090,172.27.128.32:8090,172.27.128.33:8090 --zk_root_path=/openmldb_cluster --role=ns_client
+> showtablet
+  endpoint            state           age
+-------------------------------------------
+  172.27.128.31:8541  kTabletHealthy  15d
+  172.27.128.32:8541  kTabletHealthy  15d
+  172.27.128.33:8541  kTabletHealthy  15d
+  172.27.128.37:8541  kTabletHealthy  1min
+```
+
+### Step 2. Migrating replications
+
+The command used for replication migration is `migrate`. The command format is: `migrate src_endpoint table_name partition des_endpoint` 
+
+**Once the table is created, partitions cannot be added or removed, only can be migrated. When migrating, only leader partitions can be migrated, not follower partitions**
+
+```bash
+$ ./bin/openmldb --zk_cluster=172.27.128.31:8090,172.27.128.32:8090,172.27.128.33:8090 --zk_root_path=/openmldb_cluster --role=ns_client
+> use demo_db
+> showtable
+  name    tid  pid  endpoint            role      ttl       is_alive  compress_type  offset   record_cnt  memused
+----------------------------------------------------------------------------------------------------------------------
+  flow    4   0    172.27.128.32:8541  leader    0min       yes       kNoCompress    0        0           0.000
+  flow    4   0    172.27.128.33:8541  follower  0min       yes       kNoCompress    0        0           0.000
+  flow    4   0    172.27.128.31:8541  follower  0min       yes       kNoCompress    0        0           0.000
+  flow    4   1    172.27.128.33:8541  leader    0min       yes       kNoCompress    0        0           0.000
+  flow    4   1    172.27.128.31:8541  follower  0min       yes       kNoCompress    0        0           0.000
+  flow    4   1    172.27.128.32:8541  follower  0min       yes       kNoCompress    0        0           0.000
+> migrate 172.27.128.33:8541 flow 0 172.27.128.37:8541
+> showopstatus flow
+  op_id  op_type     name  pid  status  start_time      execute_time  end_time        cur_task
+------------------------------------------------------------------------------------------------
+  51     kMigrateOP  flow  0    kDone   20180824163316  12s           20180824163328  -
+> showtable
+  name    tid  pid  endpoint            role      ttl       is_alive  compress_type  offset   record_cnt  memused
+----------------------------------------------------------------------------------------------------------------------
+  flow    4   0    172.27.128.32:8541  leader    0min       yes       kNoCompress    0        0           0.000
+  flow    4   0    172.27.128.37:8541  follower  0min       yes       kNoCompress    0        0           0.000
+  flow    4   0    172.27.128.31:8541  follower  0min       yes       kNoCompress    0        0           0.000
+  flow    4   1    172.27.128.33:8541  leader    0min       yes       kNoCompress    0        0           0.000
+  flow    4   1    172.27.128.31:8541  follower  0min       yes       kNoCompress    0        0           0.000
+  flow    4   1    172.27.128.32:8541  follower  0min       yes       kNoCompress    0        0           0.000
+```
+Note that, migrating replicatins can also be done by deleting replicatins and then adding new ones.
+
+## Scale-In
+
+Scaling in your cluster is to reduce the number of nodes in the cluster.
+
+### Step 1. Selecting the node that you want to go offline
+### Step 2. Migrating partitions on nodes that need to be taken offline to other nodes
+* Run the `showtable` command to view the partitions of a table
+* Run the `migrage` command to migrate the targeted partitions to another node. If the leader exists on the offline node, you can run the `changeleader` command to switch the leader to another node
+### Step 3. Making the targeted node offline
+- Execute `stop` command
+```bash
+sh bin/start.sh stop tablet
+```
+- If nameserver is deployed on the node, you need to disable the nameserver.
+```bash
+sh bin/start.sh stop nameserver
+```
+Note that, at least two Nameserver nodes are required to maintain high availability