[MOV] general: removes main "settings" section + move content

This commit is part of a larger work to improve the documentation's
structure. The toctree structure used to add levels to create categories
and make the structure visually more understandable.
With the new version of the documentation, plenty of these categories
are now redundant.

This commit:
- removes the main "settings" section
  - move its content to general/odoo_basics
- moves "general/mobile" content to "general"
- moves the content of the doc "developer_mode/activate" to "general"
  - it replaces the references to that doc with `:ref:` directives
    rather than `:doc:` directives
  - it is worth noting that it also fixes plenty of links that
    previously referred to "developer_mode" (the category) rather than
	"developer_mode/activate" (the document)

task-2535899

content/applications/services/planning/overview/duplicate_a_planning.rst

@@ -28,7 +28,7 @@ of re-planning it.
 
 .. note::
    Repeated shifts are generated for the next six months. A modification is possible in
-   :doc:`Developer mode </applications/general/developer_mode>`, from *Settings*.
+   :ref:`developer mode <developer-mode>`, from *Settings*.
 
       .. image:: media/rate_shift_generation.png
          :align: center

content/applications/services/project/advanced/advanced_projects.rst

@@ -100,9 +100,9 @@ Share stages across projects
 
 | In order to have a clean Kanban view that works across projects, and to avoid duplicates,
   delineate specific stages for different projects.
-| :doc:`Activate the Developer mode </applications/general/developer_mode>` then go to
-  :menuselection:`Project --> Configuration --> Stages`. Choose the respective one and add
-  the projects among which you would like to share stages.
+| Activate the :ref:`developer mode <developer-mode>` then go to :menuselection:`Project -->
+  Configuration --> Stages`. Choose the respective one and add the projects among which you would
+  like to share stages.
 
 .. image:: media/share_stages.png
    :align: center

content/applications/services/project/advanced/profitability.rst

@@ -29,9 +29,9 @@ by people according to the billable hours recorded, and even a timesheet table.
 If you do not have the Timesheets application installed
 -------------------------------------------------------
 
-| Activate the :doc:`Developer mode </applications/general/developer_mode>`. Then go to
-  *Manager Users* and enable *Analytic Accounting*. Now, go to :menuselection:`Project -->
-  Configuration --> Projects` and specify an *Analytic Account* under the needed project.
+| Activate the :ref:`developer mode <developer-mode>`. Then go to *Manager Users* and enable
+  *Analytic Accounting*. Now, go to :menuselection:`Project --> Configuration --> Projects` and
+  specify an *Analytic Account* under the needed project.
 | A *Profitability* menu to the costs and revenues of this analytic account is directly available
   in the Kanban view.
 

content/applications/services/timesheets/overview/time_off.rst

@@ -6,8 +6,8 @@ Odoo automatically timesheets on project/tasks upon time off requests. This allo
 overall control over the validation of timesheets, as it does not leave place for forgetfulness
 and questions after hours that have not been timesheeted by the employee.
 
-Activate the :doc:`Developer mode </applications/general/developer_mode>`, go to *Timesheets*,
-and change the *Project* and *Task* set by default, if you like.
+Activate the :ref:`developer mode <developer-mode>`, go to *Timesheets*, and change the *Project*
+and *Task* set by default, if you like.
 
 .. image:: media/record_time_off.png
    :align: center