docs(dashboard-widget-builder): Added section for setting thresholds. (#8250)

* docs(dashboard-widget-builder): Added section for setting thresholds.

* Update index.mdx

* Update src/docs/product/dashboards/widget-builder/index.mdx

Co-authored-by: Liza Mock <liza.mock@sentry.io>

---------

Co-authored-by: Abdullah Khan <abdullahkhan@PG9Y57YDXQ.local>
Co-authored-by: Liza Mock <liza.mock@sentry.io>

Author: 3 people

src/docs/product/dashboards/widget-builder/index.mdx

@@ -125,3 +125,10 @@ The option to sort by a y-axis is only available if you've applied a grouping to
 If you select “Custom Equation”, an extra field appears where you can enter an equation of your choice that will be applied to the query:
 
 ![Custom equation field.](widget-builder-custom-equation-field.png)
+
+## Set Thresholds
+
+The option for setting thresholds is only available for [big number visualizations](/product/dashboards/widget-builder/#big-number). Thresholds help identify problematic widgets with colored indicators. Hovering over the colored indicators at the dashboard level displays the thresholds associated with a widget. Units can be attached to thresholds for duration or rate metric types. An example metric with units would be “p95 of all transactions”.
+
+![Set Thresholds Step](widget-builder-thresholds.png)
+![Widget Coloured Indicator Hover](widget-builder-thresholds-hover.png)