docs(derived): include the MPE_DERIVED configuration, recapture at full page

The screenshots now show the whole page rather than a cropped viewport, and the
configuration table is included so the three variables in the grid can be
traced to the three rows that define them.

docs/derived-variables.md

@@ -47,7 +47,10 @@ A `WHERE` statement applies to the data set named on the `SET` statement, so in
 
 ## Configuration
 
-Derived variables are configured per table in the [MPE_DERIVED](/tables/mpe_derived/) control table, which is itself a Data Controller table - so it is maintained through the EDIT screen with the usual review and approve workflow.
+Derived variables are configured per table in the [MPE_DERIVED](/tables/mpe_derived/) control table, which is itself a Data Controller table - so it is maintained through the EDIT screen with the usual review and approve workflow. The three variables shown above, and nothing else, are the three rows below:
+
+![](./img/derived-config.png)
+
 
 Each row defines one variable: