docs: Add docs for append

See #5190

docs/content/en/functions/append.md

@@ -0,0 +1,38 @@
+---
+title: append
+description: "`append` appends one or more values to a slice and returns the resulting slice."
+date: 2018-09-14
+categories: [functions]
+menu:
+  docs:
+    parent: "functions"
+keywords: [collections]
+signature: ["COLLECTION | append VALUE [VALUE]...", "COLLECTION | append COLLECTION"]
+workson: []
+hugoversion: "0.49"
+relatedfuncs: [last,first,where,slice]
+aliases: []
+---
+
+An example appending single values:
+
+```go-html-template
+{{ $s := slice "a" "b" "c" }}
+{{ $s = $s | append "d" "e" }}
+{{/* $s now contains a []string with elements "a", "b", "c", "d", and "e" */}}
+
+```
+
+The same example appending a slice to a slice:
+
+
+```go-html-template
+{{ $s := slice "a" "b" "c" }}
+{{ $s = $s | append (slice "d" "e") }}
+```
+
+The `append` function works for all types, including `Pages`.
+
+
+
+