[ADD] purchase: Manage Blanket Orders

Original document: https://docs.google.com/document/d/1ECslIdtGVlwY6-x_Wj4GqlMpea4v5dBiiM2rPdsNji8/edit

purchase/purchases/tender/blanket_orders.rst

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+=====================
+Manage Blanket Orders
+=====================
+
+A Blanket Order is a purchase order which a customer places with its
+supplier to allow multiple delivery dates over a period of time, often
+negotiated to take advantage of predetermined pricing.
+
+Configuration
+=============
+
+For this feature to work, go to :menuselection:`Purchases -->
+Configuration --> Settings` and activate the *Purchase Agreements*
+feature
+
+.. image:: media/blanket_orders01.png
+    :align: center
+
+Start a Blanket Order
+=====================
+
+To use this feature go to :menuselection:`Purchases --> Purchase Agreements`.
+
+Once you are creating the purchase agreement, access the *Agreement
+Type* drop down menu and select *Blanket Order*.
+
+.. image:: media/blanket_orders02.png
+    :align: center
+
+You can then create your new blanket order, select your vendor, the
+product(s), agreement deadline, ordering date and delivery date.
+
+When you are satisfied with your purchase agreement, confirm it. Its
+status will change from*Draft* to *Ongoing* and a new
+*RFQs/Orders* will appear in the top right corner of the document.
+
+.. image:: media/blanket_orders03.png
+    :align: center
+
+RFQ from the Blanket Order
+--------------------------
+
+From your blanket order you can create a new quotation, Odoo will
+auto-fill the document with the product(s) from your blanket order, you
+only have to choose the quantity and confirm the order.
+
+When you will go back on the blanket order, you will see how many
+quantities you have already ordered from your blanket order.
+
+Vendors prices on your product
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+When a blanket order is validated, a new vendor line is added in your
+product. You can see which one are linked to a blanket order and give
+priority to the ones you want with the arrows on the left of the vendor
+name.
+
+Thanks to this, a blanket order can be used for automated replenishment
+(using *reordering rules* or *made to order* configuration).
+
+.. image:: media/blanket_orders04.png
+    :align: center
+
+.. tip::
+    You can also create a *Request for Quotation* from the classic RFQ
+    Menu and link them with an existing *Blanket Order*