docs(config): VARIABLE_ORDER and chained derived variables

A derived variable can now be built on one with a lower VARIABLE_ORDER, so
the pages describe the order column, the ordered rule (replacing the
self-contained one) and a chained example.

docs/tables/mpe_derived.md

@@ -17,6 +17,7 @@ The table is itself registered in [MPE_TABLES](/tables/mpe_tables/), so it is ma
  - 🔑 `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_ORDER num`: Evaluation order; a variable may reference one with a lower order
  - `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.`