docs(derived): mark the page as draft so it can merge before the release

A warning banner on both pages, and '(draft)' in the navigation title, so a
reader is not surprised when the feature is not in their installed version.
Remove both when the release carrying derived variables goes out.

mkdocs build passes; the banner and all four images render.

mkdocs.yml

@@ -32,7 +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_DERIVED (draft): tables/mpe_derived.md
     - MPE_EMAILS: tables/mpe_emails.md
     - MPE_EXCEL_CONFIG: tables/mpe_excel_config.md
     - MPE_FILTERANYTABLE: tables/mpe_filteranytable.md
@@ -59,7 +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
+    - Derived Variables (draft): derived-variables.md
     - Dynamic Cell Dropdown: dynamic-cell-dropdown.md
     - Emails: emails.md
     - Excel: excel.md