Add more docs for Feature Layer

not complete, but progress..

docs/features/people/entry/overview.rst

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+
+Overview
+========
+
+In most cases the Person-related data already lives in a (e.g. POS)
+SQL database and may be queried directly by Rattail.  This means you
+may view that data directly in the web app.
+
+All of the Person-related records may also be *imported* into Rattail
+from other systems, and this is usually the best place to get started;
+see :doc:`importing`.
+
+Once the data lives in Rattail you obviously may view it within the
+web app (so, reading from Rattail DB instead of POS DB now).  But this
+also sets the stage for other features which require such data to be
+in place; for instance :doc:`../../custorders/index`.
+
+You also may want to allow editing for some or perhaps all aspects;
+see :doc:`editing`.
+
+The data (and any changes made via editing, if applicable) may also be
+exported back to the source and/or other systems; see
+:doc:`exporting`.
+
+Sometimes you need to process a "set" of data, for instance taking an
+export file from one system and then preparing it for import to
+another system.  This is a broad topic; for a starting point see
+:doc:`batches`.
+
+Duplicate records are an ongoing problem in many systems, when it
+comes to Person-related data.  Rattail does allow merging of 2 records
+but it can be tricky; see :doc:`merging`.