Document function

Author: svilenmarkov

docs/custom-api.md

@@ -397,6 +397,7 @@ The following helper functions provided by Glance are available:
 - `sortByFloat(key string, order string, arr []JSON): []JSON`: Sorts an array of JSON objects by a float key in either ascending or descending order.
 - `sortByTime(key string, layout string, order string, arr []JSON): []JSON`: Sorts an array of JSON objects by a time key in either ascending or descending order. The format must be provided in Go's [date format](https://pkg.go.dev/time#pkg-constants).
 - `concat(strings ...string) string`: Concatenates multiple strings together.
+- `unique(key string, arr []JSON) []JSON`: Returns a unique array of JSON objects based on the given key.
 
 The following helper functions provided by Go's `text/template` are available: