More docs! w/ some emphasis on "reports" section

docs/data/reports/overview.rst

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+
+==========
+ Overview
+==========
+
+A report is conceptually just what you think it is, probably.  But there are a
+few related concepts which we'll try to briefly outline here.
+
+* report type
+* report handler
+* report output
+
+First a "report type" refers to the logic for running a specific type of
+report.  For example, "Daily Sales" or "Slow Movers" might be report types.
+This defines what sort of data is included on the report, how it is displayed
+etc.  See :doc:`custom` for how to create a new report type.
+
+Next, a "report handler" is responsible for overall logic used to orchestrate
+the running of all reports for the app.  See :doc:`handler` for more on that.
+
+Finally "report output" refers to the result of running a report.  Generally
+speaking whenever a report runs, the details are saved to the DB for
+convenience and historical record.  See :doc:`generate` for more on that.
+
+It's perhaps worth noting, that in Rattail-speak "an email is not a report" and
+vice versa.  Meaning, a report is a *generated document* and if sent via email,
+it will be an attachment as opposed to being part of the message body.  See
+also :doc:`autoemail`.