[MOV] accounting: restructuration of the accounting documentation

This commit improves the structure of the accounting documentation by
- reducing the levels necessary to reach the information
- grouping some docs
- moving content to their parent category page (content in all levels)
- renaming some sections to reflect the structure of the app better

task-3330093

closes odoo/documentation#4470

Signed-off-by: Castillo Jonathan (jcs) <jcs@odoo.com>

content/applications/finance/accounting/customer_invoices/snailmail.rst

@@ -0,0 +1,57 @@
+.. _customer_invoices/snailmail:
+
+=========
+Snailmail
+=========
+
+Sending direct mail can be an effective strategy for grabbing people's attention, especially when
+their email inboxes are overflowing. With Odoo, you have the ability to send invoices and follow-up
+reports through postal mail worldwide, all from within your database.
+
+Configuration
+=============
+
+Go to :menuselection:`Accounting --> Configuration --> Settings --> Customer invoices` section to
+activate :guilabel:`Snailmail`.
+
+To make it a by-default feature, select :guilabel:`Send by Post` in the :guilabel:`Default Sending
+Options` section.
+
+.. image:: snailmail/setup-snailmail.png
+   :align: center
+   :alt: Under settings enable the snailmail feature in Odoo Accounting
+
+Send invoices by post
+=====================
+
+Open your invoice, click on :guilabel:`Send & Print` and select :guilabel:`Send by Post`. Make sure
+your customer’s address is set correctly, including the country, before sending the letter.
+
+.. important::
+   Your document must respect the following rules to pass the validation before being sent:
+
+   - Margins must be **5 mm** on all sides. As Odoo forces the outer margins by filling them with
+     white before sending the snailmail, it can results in the user's custom being cut off if it
+     protrudes into the margins. To check the margins, activate the :ref:`developer mode
+     <developer-mode>`, go to :menuselection:`General Settings --> Technical --> Reporting
+     section: Paper Format`.
+   - A square of **15mm by 15mm** on the bottom left corner has to stay clear.
+   - The postage area has to stay clear (click :download:`here <snailmail/snailmail-template.pdf>`
+     to get more info about the area).
+   - Pingen (Odoo Snailmail service provider) scans the area to process the address, so if something
+     gets written outside the area, it is not counted as part of the address.
+
+Pricing
+=======
+
+Snailmail is an :doc:`/applications/general/in_app_purchase` service that requires prepaid stamps
+(=credits) to work. Sending one document consumes one stamp.
+
+To buy stamps, go to :menuselection:`Accounting --> Configuration --> Settings --> Customer
+invoices: Snailmail`, click on :guilabel:`Buy credits`, or go to :menuselection:`Settings --> In-App
+Purchases: Odoo IAP`, and click on :guilabel:`View my Services`.
+
+.. tip::
+   - If you are on Odoo Online and have the Enterprise version, you benefit from free trial credits
+     to test the feature.
+   - Click `here <https://iap.odoo.com/privacy#header_4>`_ to know about our *Privacy Policy*.