[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/finance/accounting/fiscal_localizations/localizations/france.rst

@@ -74,7 +74,7 @@ To get the certification just follow the following steps:
 .. note::
    * To install the module in any system created before
      December 18th 2017, you should update the modules list.
-     To do so, activate the :doc:`Developer mode </applications/general/developer_mode>`.
+     To do so, activate the :ref:`developer mode <developer-mode>`.
      Then go to the *Apps* menu and press *Update Modules List* in the top-menu.
    * In case you run Odoo on-premise, you need to update your installation
      and restart your server beforehand.
@@ -156,8 +156,8 @@ Invoicing and Accounting apps.
    it will be counted in the very next closing.
 
 .. tip:: For test & audit purposes such closings can be manually generated in the
-   :doc:`Developer mode </applications/general/developer_mode>`. Then go to
-   :menuselection:`Settings --> Technical --> Automation --> Scheduled Actions`.
+   :ref:`developer mode <developer-mode>`. Then go to :menuselection:`Settings -->
+   Technical --> Automation --> Scheduled Actions`.
 
 
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