[Docs] Add section for assigning colors to terms (elastic#199241)

## Summary

Added a section with instructions for assigning colors to terms in a
table. The technical preview warning will be removed in GA.

Rel: elastic#189895
Closes: [elastic#559](elastic/platform-docs-team#559)

---------

Co-authored-by: florent-leborgne <[email protected]>

docs/user/dashboard/lens.asciidoc

@@ -96,13 +96,37 @@ All columns that belong to the same layer pane group are sorted in the table.
 
 * *Text alignment* — Aligns the values in the cell to the *Left*, *Center*, or *Right*.
 
+* *Color by value* — Applies color to the cell or text values. To change the color, click the *Edit colors* icon. 
+
 * *Hide column* — Hides the column for the field.
 
 * *Directly filter on click* — Turns column values into clickable links that allow you to filter or drill down into the data.
 
 * *Summary row* — Adds a row that displays the summary value. When specified, allows you to enter a *Summary label*.
 
-* *Color by value* — Applies color to the cell or text values. To change the color, click *Edit*. 
+[float]
+[[assign-colors-to-terms]]
+===== Assign colors to terms
+
+preview::[]
+
+For term-based metrics, assign a color to each term with color mapping. 
+
+. Create a custom table.
+
+. In the layer pane, select a *Rows* or *Metrics* field. 
+
+. In the *Color by value* option, select *Cell* or *Text*.
+
+. Click the *Edit colors* icon.
+
+. Toggle the button to use the Color Mapping feature.
+
+. Select a color palette and mode.
+
+. Click *Add assignment* to assign a color to a specific term, or click *Add all unassigned terms* to assign colors to all terms. Assigning colors to dates is unsupported.
+
+. Configure color assignments. You can also select whether unassigned terms should be mapped to the selected color palette or a single color. 
 
 [float]
 [[drag-and-drop-keyboard-navigation]]