[ADD] purchase: How to purchase partially at two vendors for the same purchase tenders?

https://docs.google.com/document/d/1-d3hjWIfUm5Xoz7VIMIqAcS1rpDP52Rty9gLWetY6Rk/edit#heading=h.2mjdizz26oox

purchase/purchases/tender/partial_purchase.rst

@@ -1,3 +1,77 @@
 =======================================================================
 How to purchase partially at two vendors for the same purchase tenders?
-=======================================================================
+=======================================================================
+
+For some Purchase Tenders (PT), you might sometimes want to be able to
+select only a part of some of the offers you received. In Odoo, this is
+made possible through the advanced mode of the **Purchase** module.
+
+.. note::
+    If you want to know how to handle a simple **Purchase Tender**, 
+    read the document on :doc:`manage_multiple_offers`.
+
+Configuration
+-------------
+
+Install the Purchase Management module
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+From the **Apps** menu, install the **Purchase Management** app.
+
+.. image:: media/partial_purchase02.png
+    :align: center
+
+Activating the Purchase Tender and Purchase Tender advanced mode
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+In order to be able to select elements of an offer, you must activate
+the advanced mode.
+
+To do so, go into the **Purchases** module, open the **Configuration** menu and
+click on **Settings**.
+
+In the **Calls for Tenders** section, tick the option **Allow using call
+for tenders to get quotes from multiple suppliers(...)**, and in the
+**Advanced Calls for Tenders** section, tick the option **Advanced call
+for tender (...)** then click on **Apply**.
+
+.. image:: media/partial_purchase01.png
+    :align: center
+
+Selecting elements of a RFQ/Bid
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+Go to :menuselection:`Purchase --> Purchase Tenders`. 
+Create a purchase tender containing
+several products, and follow the usual sequence all 
+the way to the **Bid Selection** status.
+
+When you closed the call, click on **Choose Product Lines** to access the
+list of products and the bids received for all of them.
+
+.. image:: media/partial_purchase04.png
+    :align: center
+
+Unroll the list of offers you received for each product, and click on
+the *v* symbol (**Confirm order**) next to the offers you wish to proceed
+with. The lines for which you've confirmed the order turn blue. When
+you're finished, click on **Generate PO** to create a purchase order for
+each product and supplier.
+
+.. image:: media/partial_purchase05.png
+    :align: center
+
+When you come back to you purchase tender, you can see that the status has switched
+to **PO Created** and that the **Requests for Quotations** now have a status
+of **Purchase Order** or **Cancelled**.
+
+.. image:: media/partial_purchase03.png
+    :align: center
+
+.. tip::
+    From there, follow the documentation :doc:`../../overview/process/from_po_to_invoice`
+    to proceed with the delivery and invoicing.
+
+.. seealso::
+    * :doc:`manage_multiple_offers`
+    * :doc:`../../overview/process/from_po_to_invoice`