feat(dashboards): Update Widget Builder doc to include Big Number Equations (#11470)

* Update index.mdx

* add link to equation

Author: harshithadurai

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

@@ -44,7 +44,7 @@ Table charts display the results of a [Discover Query](/product/explore/discover
 
 ### Big Number
 
-A big number visualization displays the current value of a single function. This visualization is well suited for high-level aggregates. An example scenario would be “p95 of all transactions”.
+A big number visualization displays the current value of a single function or [equation](/product/explore/discover-queries/query-builder/query-equations/). This visualization is well suited for high-level aggregates. An example scenario would be “p95 of all transactions”.
 
 ## Choose Your Dataset