Add some basic docs for data import/export

docs/data/importing/overview.rst

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+
+Overview
+========
+
+Rattail offers a framework which may be used for (presumably) all data
+import and export tasks, regardless of which systems are involved, or
+direction of data flow between them.
+
+In other words it's not just for getting data into and out of a
+Rattail DB, but e.g. might be used for exporting data from your POS to
+an online mailing list, etc.
+
+See :doc:`concepts` for more background on the framework.
+
+The framework offers the following features:
+
+ * "dry run" mode to check things out before committing changes
+ * "warnings" mode which sends email with data diffs, e.g. when you expect no changes
+ * adjustable "batch size" for grouping changes when submitting to local (target) system
+ * full command line support for above, plus "max" changes to apply, show progress, etc.
+ * core code is optimized to run quickly, e.g. by fetching all data up-front
+ * new importers may be created simply / cleanly / according to existing patterns
+ * new importers may extend / replace core functionality as needed
+
+See also the API docs for :mod:`rattail:rattail.importing` (and sub-modules).