[ADD] accounting: Generate recurring vendor bills

Original doc: https://docs.google.com/document/d/1_rv0qp2w79_emNDDZls40mgdDpXR-PeEoJM7HeLWg38/edit

accounting/payables/supplier_bills.rst

@@ -7,3 +7,4 @@ Vendor Bills
 
    supplier_bills/manage
    supplier_bills/bills_or_receipts
+   supplier_bills/recurring_vendor_bills

accounting/payables/supplier_bills/recurring_vendor_bills.rst

@@ -0,0 +1,67 @@
+===============================
+Generate recurring vendor bills
+===============================
+
+You may receive vendor bills for a fixed amount on a regular basis: for
+instance, for water, gas, electricity or phone bills. For such cases,
+the recurring vendor bills feature will save you a lot of time. Indeed,
+all you need to do is to create a bill once, set a frequency of
+recurrence and the rest will follow. Your next bills will automatically
+appear in your feed.
+
+How to activate the recurring vendor bills feature
+--------------------------------------------------
+
+For this feature to work, go in
+:menuselection:`Invoicing/Accounting --> Configuration --> Settings` and
+tick the box in front of Recurring Vendor Bills to activate
+the feature. Finally, do not forget to save your settings from the
+top-left corner of your screen.
+
+.. image:: media/recurring_vendor_bills01.png
+    :align: center
+
+Generate recurring vendor bills
+-------------------------------
+
+You can either select an existing bill or create a new one for this
+purpose. Once you are editing the vendor bill, access the "other info"
+tab. On the bottom right corner, you can see that the option "recurring
+bills" is available. Tick the box and set the frequency you would like
+to have. It can be every (other) day, week, month, or year. Finally, do
+not forget to save the bill.
+
+.. image:: media/recurring_vendor_bills02.png
+    :align: center
+
+.. tip::
+    The recurring bills will only be generated if the bill is
+    validated. If the bill is cancelled, recurring bills won’t be generated
+    anymore.
+
+The recurrency depends on the bill date and on the recurrency defined. In
+the example below, the bill date is on the 13th of January and the recurrency
+set is "every 1 month(s)". In this case, the next bill will be generated on
+the 13th of February.
+
+.. image:: media/recurring_vendor_bills03.png
+    :align: center
+
+Where will the next bills appear ?
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+The next recurring bills will appear automatically on your vendor bill
+list once their recurrence date is reached. They will be created in
+draft, you will just have to verify and validate them.
+
+.. image:: media/recurring_vendor_bills04.png
+    :align: center
+
+How can I end the recurrence ?
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+To end the recurrence, simply go back on the original bill generating
+the recurrence and untick the box "Recurring Bills".
+
+.. image:: media/recurring_vendor_bills05.png
+    :align: center