docs: add basic docs for batch feature

docs/narr/batch/index.rst

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+
+Batches
+=======
+
+A :term:`batch` is essentially a temporary table with row data of a
+certain type, which can be previewed and finalized before "executing"
+it.  For instance it might be used to handle import of CSV files
+uploaded by the user, but there are many other use cases.
+
+.. note::
+
+   While a batch may be used to import data, these are different
+   concepts.  A true data import process is normally automated, with
+   no preview tooling per se.  A batch normally has preview tooling
+   but its execution may or may not import data per se.
+
+   For true data import see :doc:`wuttasync:index`.
+
+The batch concept (as used here) comes from Rattail; see also
+:doc:`rattail:narr/batches`.
+
+Each batch will be of a certain :term:`batch type`.  Each instance of
+the :term:`app` will have just one :term:`batch handler` responsible
+for each batch type.
+
+Batch data may come from CSV file, SQL query, external API etc.  In
+some cases (e.g. creating a purchase order, or counting inventory) the
+data is added over time by the user.
+
+The user can review all data in the batch before executing.  Some
+batches may allow the user to add/remove/modify rows (or other data)
+in the batch prior to execution.
+
+Upon execution the batch handler's
+:meth:`~wuttjamaican.batch.BatchHandler.execute()` method is invoked,
+so that will determine what is done with the batch data.  Once a batch
+is executed, it is "frozen" with no further modifications allowed to
+it.
+
+
+.. toctree::
+   :maxdepth: 3
+
+   model
+   handlers
+
+
+.. image:: https://rattailproject.org/images/batch-pattern.png
+