[DOCS] add replace yb-master docs

docs/content/latest/troubleshoot/cluster/replace_master.md

@@ -0,0 +1,73 @@
+---
+title: Replace a failed YB-Master
+headerTitle: Replace a failed YB-Master
+linkTitle: Replace a failed YB-Master
+description: Steps to replace a failed YB-Master in a YugabyteDB cluster.
+aliases:
+  - /troubleshoot/cluster/replace-master/
+  - /latest/troubleshoot/cluster/replace-master/
+menu:
+  latest:
+    identifier: replace-failed-master
+    parent: troubleshoot-cluster
+    weight: 831
+isTocNested: true
+showAsideToc: true
+---
+
+To replace a failed YB-Master server in a YugabyteDB cluster, follow these steps:
+
+For the steps below, the examples use the following scenario:
+
+- The cluster includes three `yb-master` servers: `M1`, `M2`, `M3`.
+- YB-Master server `M1` failed and needs to be replaced.
+- A new YB-Master server (`M4`) will replace `M1`.
+- The default master RPC port is `7100`
+
+
+1. Start the new (replacement) YB-Master server in standby mode. 
+
+To start `yb-master` in standby mode, set the `--master_addresses` flag to an empty string (`""`).
+
+```sh
+./bin/yb-master --master_addresses=""
+```
+
+2. Add the new YB-Master server into the existing cluster.
+
+To add the new YB-Master server, run the [`yb-admin change_master_config ADD_SERVER`](../../admin/cli/yb-admin/#change-master-config) command.
+
+```sh
+./bin/yb-admin -master_addresses M1:7100,M2:7100,M3:7100 change_master_config ADD_SERVER M4 7100
+```
+
+3. Remove the old YB-Master server from the cluster.
+
+To remove the failed YB-Master server from the cluster, use the [`yb-admin change_master_config REMOVE_SERVER`](../../admin/yb-admin/#change-master-config`) command.
+
+```sh
+./yb-admin -master_addresses M1:7100,M2:7100,M3:7100,M4:7100 change_master_config REMOVE_SERVER M1 7100
+```
+{{< note title="Note" >}}
+
+Make sure to specify all YB-Master addresses which will prevent the new YB-Master server from unexpectedly becoming the leader.
+
+{{< /note >}}
+## Validate cluster
+
+Validate that your set of masters is now `M2`, `M3` and `M4` using:
+```bash
+yb-admin -master_addresses M2:7100,M3:7100,M4:7100 list_all_masters
+```
+
+Until [#1542](https://github.com/yugabyte/yugabyte-db/issues/1542) is implemented, the TS will by default only know of 
+whatever masters are encoded in the `--tserver_master_addrs` flag that they are started with. 
+
+If any one of those masters is still part of the active quorum, then they will propagate the new master quorum over via heartbeats. 
+If, however, none of the current masters are present in the TS flag, then the TS will not be able to join the cluster! 
+
+So it is important to make sure to update `--tserver_master_addrs` on every TS to the new set of master addresses, `M2:7100,M3:7100,M4:7100`!
+
+{{< note title="Note" >}}
+If the master you wish to replace is already dead (eg: VM was terminated), you might want to first do the `REMOVE` step, then do the `ADD` step.
+{{< /note >}}