Add TCM failover docs (#4485)

Resolves #4250 

Co-authored-by: Elena Shebunyaeva <elena.shebunyaeva@gmail.com>

doc/tooling/tcm/tcm_cluster_management/index.rst

@@ -17,4 +17,5 @@ to learn how to perform various management operations on Tarantool clusters from
     tcm_cluster_state
     tcm_cluster_config
     tcm_cluster_users
-    tcm_cluster_metrics
+    tcm_cluster_metrics
+    tcm_supervised_failover

doc/tooling/tcm/tcm_cluster_management/tcm_supervised_failover.rst

@@ -0,0 +1,72 @@
+..  _tcm_supervised_failover:
+
+Using supervised failover
+=========================
+
+..  include:: ../index.rst
+    :start-after: ee_note_tcm_start
+    :end-before: ee_note_tcm_end
+
+For Tarantool clusters that use :ref:`supervised failover <repl_supervised_failover>`,
+|tcm_full_name| offers tools for interaction with external failover coordinators from its web interface.
+
+The tools for using supervised failover are located on the **Failovers** page
+available from the **Actions** menu on the cluster stateboard.
+
+.. note::
+
+    |tcm| can interact with failover coordinators that are already running.
+    There is no way to start or stop coordinators from |tcm|.
+
+..  _tcm_supervised_failover_view:
+
+Viewing failover coordinators
+-----------------------------
+
+To view failover coordinators running on the cluster, go to the **Failovers** tab.
+On this tab, you can see the information about all Tarantool instances that the cluster
+uses as failover coordinators. The information includes:
+
+-   Current coordinator status -- ``Active`` or ``Not active``
+-   ``PID`` -- process ID
+-   ``Hostname`` -- the host on which the coordinator is running
+-   ``UUID`` -- the coordinator ID
+-   ``Term`` -- a value that defines the order in which coordinators become active
+    (take the lock) over time.
+
+
+..  _tcm_supervised_failover_commands:
+
+Executing failover commands
+---------------------------
+
+To send a failover command to a coordinator, go to the **Commands** tab and click **Add**.
+Then, provide the command description in the YAML format. It can include the following
+fields:
+
+-   ``command`` -- the command name. Possible value: ``switch`` -- switch master
+    in a replica set.
+-   ``new_master`` -- the name of the instance to make the new master.
+-   ``timeout`` -- the command execution timeout.
+
+Example:
+
+.. code-block:: yaml
+
+    command: switch
+    new_master: instance-002
+    timeout: 30
+
+After entering the command, click **Save** to send the command for execution.
+
+Tarantool assigns an id to the command and waits for the active coordinator to process the command.
+
+All failover commands executed on the cluster are shown on the **Commands** tab with
+their ids and statuses. A command can have the following statuses:
+
+-   ``taken`` -- a failover coordinator has started the command execution.
+-   ``success`` -- the command has completed successfully.
+-   ``failed`` -- an error occurred during the command execution.
+    A short error description is shown in the **Reason** field.
+
+To see the command execution details, click this command in the list.