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.

mkdocs.yml

@@ -32,6 +32,7 @@ nav:
     - MPE_DATASTATUS_TABS: tables/mpe_datastatus_tabs.md
     - MPE_DATADICTIONARY: tables/mpe_datadictionary.md
     - MPE_DATALOADS: tables/mpe_dataloads.md
+    - MPE_DERIVED: tables/mpe_derived.md
     - MPE_EMAILS: tables/mpe_emails.md
     - MPE_EXCEL_CONFIG: tables/mpe_excel_config.md
     - MPE_FILTERANYTABLE: tables/mpe_filteranytable.md
@@ -58,6 +59,7 @@ nav:
     - CAS Tables: cas-tables.md
     - Column Level Security: column-level-security.md
     - Dates / Datetimes: dcc-dates.md
+    - Derived Variables: derived-variables.md
     - Dynamic Cell Dropdown: dynamic-cell-dropdown.md
     - Emails: emails.md
     - Excel: excel.md