[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/general/odoo_basics/users.rst

@@ -39,8 +39,7 @@ the user. The user must click on it to accept the invitation and create a login.
    `pricing page <https://www.odoo.com/pricing>`_
    for more information.
 
-With the :doc:`Developer mode </applications/general/developer_mode>` activated, *User Types* can
-be selected.
+With the :ref:`developer mode <developer-mode>` activated, *User Types* can be selected.
 
 .. image:: users/user-type.png
    :align: center
@@ -121,8 +120,8 @@ can reaccess your database using your new password.
 Access Rights in detail
 =======================
 
-Activate the :doc:`Developer mode </applications/general/developer_mode>`, then go to
-:menuselection:`Settings --> Users & Companies --> Groups`.
+Activate the :ref:`developer mode  <developer-mode>`, then go to :menuselection:`Settings --> Users
+& Companies --> Groups`.
 
 Groups
 ------
@@ -203,4 +202,4 @@ the user can have access.
 
 .. seealso::
    - :doc:`../multi_companies`
-   - :doc:`../../settings/users_and_features`
+   - :doc:`../../general`