Add strings.HasSuffix documentation

I tracked this down *via* <https://discourse.gohugo.io/t/solved-add-hassuffix-function/13160>. It came in handy for me; may it do so for others as well.

content/en/functions/strings.HasSuffix.md

@@ -0,0 +1,23 @@
+---
+title: strings.HasSuffix
+description: Determine whether or not a given string ends with the provided trailing suffix string.
+godocref:
+date: 2019-08-13
+publishdate: 2019-08-13
+lastmod: 2019-08-13
+categories: [functions]
+menu:
+  docs:
+    parent: "functions"
+keywords: [strings]
+signature: ["strings.HasSuffix STRING SUFFIX"]
+workson: []
+hugoversion:
+relatedfuncs: [hasPrefix]
+deprecated: false
+aliases: []
+---
+
+    {{ $pdfPath := "/path/to/some.pdf" }}
+    {{ strings.HasSuffix $pdfPath "pdf" }} → true
+    {{ strings.HasSuffix $pdfPath "txt" }} → false