docs(config): derived variables in the EDIT screen

New Configuration page explaining derived (computed) variables: what they
are, how to configure them in MPE_DERIVED, how filtering works, the rules
and limits (expressions only, self-contained, read-only, not stored), and
the data-exposure and performance notes.

New MPE_DERIVED table page, registered in the Table Guide nav.

docs/tables/mpe_derived.md

@@ -0,0 +1,24 @@
+---
+layout: article
+title: MPE_DERIVED
+description: The MPE_DERIVED table is used to configure computed columns that appear in the EDIT screen and can be filtered on without being stored in the table.
+---
+
+# MPE_DERIVED
+
+The MPE_DERIVED table is used to configure derived (computed) variables - columns that are created when a table is loaded into the EDIT screen, so that they can be displayed and filtered on without being stored.  More details are available in the [user guide](/derived-variables/)
+
+The table is itself registered in [MPE_TABLES](/tables/mpe_tables/), so it is maintained through the EDIT screen with the usual review and approve workflow.
+
+## Columns
+
+ - `TX_FROM num`: SCD2 open datetime
+ - 🔑 `TX_TO num`: SCD2 close datetime
+ - 🔑 `LIBREF char(8)`: The libref of the target table
+ - 🔑 `DSN char(32)`: The target table
+ - 🔑 `VARIABLE_NM char(32)`: The name of the derived column
+ - `VARIABLE_TYPE char(1)`: `C` for character, `N` for numeric
+ - `VARIABLE_LENGTH num`: Character length, or numeric length (1-8)
+ - `VARIABLE_FORMAT char(32)`: Optional SAS format, eg `DATE9.`
+ - `VARIABLE_LOGIC char(4000)`: The SAS expression that produces the value
+ - `ACTIVE num`: Set to 1 to apply the variable