[Docs] Configuring catalog promotions batch sizes explained

docs/cookbook/promotions/catalog-promotion-batch-sizes.rst

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+How to customize catalog promotion batches size?
+================================================
+
+In case of looking for improving performance of processing your `catalog promotions`
+
+Change default batch size:
+----------------------------
+
+By default `batch_size` in Sylius is fixed at processing `100` variants per batch.
+In order if you want to increase or decrease it's value, you have to overwrite it.
+You can achieve that by adding to your `_sylius.yaml` configuration file this config with changed `batch_size` value:
+
+.. code-block:: yaml
+
+    sylius_core:
+        catalog_promotions:
+            batch_size: 100
+
+Then your batch size should be fixed to passed value.