feat: Array sum filter (#661)

* feat(filters): add array sum filter

* docs(filters): array sum filter

* docs: update versoin number in source/filters/sum.md

---------

Co-authored-by: Jun Yang <[email protected]>

docs/source/_data/sidebar.yml

@@ -70,6 +70,7 @@ filters:
   strip: strip.html
   strip_html: strip_html.html
   strip_newlines: strip_newlines.html
+  sum: sum.html
   times: times.html
   truncate: truncate.html
   truncatewords: truncatewords.html

docs/source/filters/sum.md

@@ -0,0 +1,22 @@
+---
+title: sum
+---
+
+{% since %}v10.10.0{% endsince %}
+
+Computes the sum of all the numbers in an array.
+An optional argument specifies which property of the array's items to sum up.
+
+In this example, assume the object `cart.products` contains an array of all products in the cart of a website.
+Assume each cart product has a `qty` property that gives the count of that product instance in the cart.
+Using the `sum` filter we can calculate the total number of products in the cart.
+
+Input
+```liquid
+The cart has {{ order.products | sum: "qty" }} products.
+```
+
+Output
+```text
+The cart has 7 products.
+```