Add some more "handler" docs

app handler, batch handlers, native batch types, people handler

docs/data/batch/native/labels.rst

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+
+Label Batch
+===========
+
+**Batch Type Key:** ``labels``
+
+**Purpose:** Print item labels / shelf tags for a set of products.
+
+**Default Handler:** :class:`~rattail:rattail.batch.labels.LabelBatchHandler`
+
+Data for this batch can come directly from file, or from a
+:doc:`handheld` etc.  It can contain a list of items and various label
+printing attributes for each.
+
+Executing this batch may send a job(s) directly to your dedicated
+label printer if you have one.  Or it can generate an Excel file for
+use with BarTender or other software, etc.