docs(7.19): publish derived variables - banners off, nav titles clean, roadmap entry

Held until the release lands: the draft banner on both derived pages, the
(draft) suffix in the nav, and the Delivered Features entry they were waiting
for. Merge this with, or just after, dc/dc#348.

docs/roadmap.md

@@ -45,6 +45,10 @@ Described [here](https://docs.datacontroller.io/api/).
 
 Below are some examples of Features that have been requested (and delivered) into Data Controller.
 
+### Derived Variables
+
+Columns that do not exist in the table - a banding, a concatenation, a calculation - can be configured per table and appear in the EDIT screen, read-only and filterable, without changing the table or the load process. Nothing derived is stored.  See [docs](/derived-variables)
+
 ### Column Level Security
 
 The columns that can be VIEWed or EDITed are now configurable.  See [docs](/column-level-security)