[IMP] purchase: remove easy docs and restructure kept docs (#814)

Most of the docs removed were easy/basic and explained under Odoo Learn.
As many docs were removed, it was necessary to restructre and rename the docs that I kept.
Kept docs will be updated next.

Co-authored-by: Xavier (xpl) <“xpl@odoo.com”>

purchase/products/reordering.rst

@@ -0,0 +1,51 @@
+=================================================
+Don’t run out of stock thanks to reordering rules
+=================================================
+
+To make sure you never run out of stock, you might want to define
+*Reordering Rules* on products. Thanks to reordering rules, Odoo will
+help you to replenish your stock when it reaches a minimum stock level.
+
+Set a reordering rule
+=====================
+
+Open a stockable product, you'll find the *Reordering Rules* tab.
+
+.. note::
+  You can also access all your reordering rules from
+  :menuselection:`Inventory --> Master Data --> Reordering Rules`.
+
+.. image:: media/setup_stock_rule01.png
+  :align: center
+
+Once in the next menu, you can create the reordering rules linked to
+your product. From now on, every time this product reaches the minimum
+quantity it will replenish your stock to your **maximum quantity**.
+
+.. image:: media/setup_stock_rule02.png
+  :align: center
+
+.. tip::
+  The replenishments will take place when the scheduler in the
+  *Inventory* module runs. By default in Odoo, the schedulers will run
+  every night at 12:00PM.
+
+  See also: :doc:`../../../inventory/management/planning/schedulers`
+
+From now on, every time a product with a reordering rule reaches the
+minimum stock, the system will automatically see how to fulfill that
+need based on the product configuration (e.g create an RfQ, create an
+MO, etc.).
+
+Troubleshooting
+===============
+
+For your reordering rules to work, your product needs to be correctly
+configured. If you are using *Buy* route, you need a vendor on the
+product. If you are using *Manufacturing* route you will been a BoM
+on the product. If the product is not fully configured or there is a
+mistake in your routes, a next activity will be logged on the product
+informing you there is an issue with the configuration.
+
+.. image:: media/setup_stock_rule03.png
+  :align: center