Add some more "handler" docs

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

docs/data/batch/native/vendor_catalog.rst

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+
+Vendor Catalog Batch
+====================
+
+**Batch Type Key:** ``vendor_catalog``
+
+**Purpose:** Allows updating product costs from new catalog data
+
+**Default Handler:** :class:`~rattail:rattail.batch.vendorcatalog.VendorCatalogHandler`
+
+Data for this type of batch usually comes from a spreadsheet or
+similar file, as obtained directly from the vendor.  You can also
+define a "common" format to use, in which case you would convert each
+vendor file to that format.  (That is to cut down on the number of
+file parsers needed.)
+
+In any case executing this batch will just update Rattail by default;
+you may want a custom handler to update your POS system instead.