docs(derived): publish the page, and the images for the computed-column styling

The screenshots were taken on the mocked estate, before setvars began marking
computed columns with an fx badge, a tinted header and a left edge where the
derived block starts.  They are re-taken against a real SAS backend, so the
images show what ships, and the page describes the marking.

The draft banner comes off both pages, the (draft) suffix off both nav entries,
and the roadmap gains its Delivered Features entry - all held until the release
lands.

docs/derived-variables.md

@@ -6,11 +6,6 @@ description: Derived variables are computed columns that appear in the EDIT scre
 
 # Derived Variables
 
-!!! warning "Draft - not in a release yet"
-    This page describes a feature that is in development. It is published early so
-    the wording and the screenshots can be reviewed; the behaviour described here
-    ships in the next release.
-
 A derived variable is a column that does not exist in the target table, but is computed when the table is loaded into the [EDIT screen](/dc-userguide/#editor). It is read-only, it appears after the physical columns, and it can be filtered on like any other column.
 
 Derived variables are useful when users need to work with a value that is a function of the stored data - a banding, a concatenation, a calculation - without changing the table, the load process, or the ETL.
@@ -23,6 +18,8 @@ The table below is shown in the EDIT screen. `LINE_VALUE` is `QTY * PRICE` and `
 
 ![](./img/derived-grid.png)
 
+The computed columns are marked apart from the physical ones: a tinted header with an `fx` badge, tinted italic cells, and a left edge where the derived block begins. They are read-only, and the marking says so - a column that is locked by a rule or by column level security is dimmed instead, which is a different message from "computed here". Hovering a header shows the variable's label, or its name and expression where no label was configured.
+
 Because the derived columns are created before the filter is applied, they can be filtered like any other column. Here `REGION = 'West'` selects the two western states, and the filter bar names the derived variable:
 
 ![](./img/derived-filter-region.png)