[ADD] project: new doc about sub-tasks

taskid-4066254

closes odoo/documentation#10290

Signed-off-by: Audrey Vandromme (auva) <auva@odoo.com>

content/applications/services/project/tasks.rst

@@ -10,3 +10,4 @@ Task management
    tasks/task_stages_statuses
    tasks/task_creation
    tasks/recurring_tasks
+   tasks/sub-tasks

content/applications/services/project/tasks/sub-tasks.rst

@@ -0,0 +1,51 @@
+=========
+Sub-tasks
+=========
+
+When handling project tasks, you can often benefit from splitting the workload into smaller
+**sub-tasks**, making it easier to track progress and manage the work.
+
+Odoo’s sub-task feature is particularly useful in some of the following cases:
+
+- The task workload is too big to handle in one record.
+- There’s a clear sequence of steps to follow.
+- Some parts of the workload fall under the scope of different projects and/or assignees.
+
+The original task to which sub-tasks are linked is referred to as the **parent task**. The sub-tasks
+of the parent task are known as **child tasks**.
+
+Creating sub-tasks
+==================
+
+To create a sub-task:
+
+#. Open the Project app, then go into your desired project.
+#. Click the task where you want to add the sub-task, then click the :guilabel:`Sub-tasks` tab.
+#. Click :guilabel:`Add a line`, then fill in the :guilabel:`Title`.
+#. Click the :icon:`fa-cloud-upload` (:guilabel:`cloud`) icon to save the task manually.
+
+.. tip::
+   You can edit several fields of a sub-task directly from the parent task’s sub-task tab without
+   opening the individual sub-task. By default, those are: :guilabel:`Title`, :guilabel:`Priority`,
+   :guilabel:`Status`, and :guilabel:`Assignees`. Use the :icon:`fa-sliders` (:guilabel:`sliders`)
+   icon to add or remove fields.
+
+Once the new sub-task is created, the following changes occur in the parent task:
+
+- A **smart button** displays the total number of sub-tasks, as well as the number of closed
+  (**done** or **canceled**) sub-tasks. Click the smart button to access the list of the sub-tasks.
+- The :guilabel:`Allocated time` displays the amount of time allocated to the parent task, as well
+  as time allocated to the sub-tasks. This can be used to ensure that the time allocated to
+  sub-tasks does not exceed time allocated to the parent task.
+- The :guilabel:`Timesheets` tab includes a breakdown of time spent on the parent task as well as
+  sub-tasks.
+
+Click :guilabel:`View` to open the sub-task you created. The sub-task form is identical to the
+:ref:`task form <task_creation/task-configuration>`.
+
+.. tip::
+
+ - A **smart button** allows to navigate back to the :guilabel:`Parent task`.
+ - The :guilabel:`Sub-tasks` tab allows for further creation of sub-tasks.
+ - Selecting a :guilabel:`Project` will trigger this sub-task to be displayed in the Kanban view of
+   the selected project.