[ADD] inventory: What's the difference between lots and serial numbers?

https://docs.google.com/document/d/1tYQjuKEac1frOlmwDf7ZCHLI1CWViypTKA_mYN6Ny4A/edit

inventory/lots_serial_numbers/differences.rst

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+======================================================
+What's the difference between lots and serial numbers?
+======================================================
+
+Introduction
+============
+
+In Odoo, lots and serial numbers have similarities in their functional
+system but are different in their behavior. They are both managed within
+the **Inventory**, **Purchases** and **Sales** app.
+
+**Lots** correspond to a certain number of products you received and store
+altogether in one single pack.
+
+**Serial numbers** are identification numbers given to one product in
+particular, to allow to track the history of the item from reception to
+delivery and after-sales.
+
+When to use
+-----------
+
+**Lots** are interesting for products you receive in great quantity and for
+which a lot number can help in reportings, quality controls, or any
+other info. Lots will help identify a number of pieces having for
+instance a production fault. It can be useful for a batch production of
+clothes or food.
+
+**Serial numbers** are interesting for items that could require after-sales
+service, such as smartphones, laptops, fridges, and any electronic
+devices. You could use the manufacturer’s serial number or your own,
+depending on the way you manage these products
+
+When not to use
+---------------
+
+Storing consumable products such as kitchen roll, toilet paper, pens and
+paper blocks in lots would make no sense at all, as there are very few
+chances that you can return them for production fault.
+
+On the other hand, giving a serial number to every product is a
+time-consuming task that will have a purpose only in the case of items
+that have a warranty and/or after-sales services. Putting a serial
+number on bread, for instance, makes no sense at all.
+
+.. seealso::
+    * :doc:`serial_numbers`
+    * :doc:`lots`

inventory/lots_serial_numbers/lots.rst

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+=========================================
+How to manage lots of identical products?
+=========================================

inventory/lots_serial_numbers/serial_numbers.rst

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+================================
+How to work with serial numbers?
+================================