docs: Document group

Fixes gohugoio/hugoDocs#597

docs/content/en/functions/group.md

@@ -0,0 +1,37 @@
+---
+title: group
+description: "`group` groups a list of pages."
+date: 2018-09-14
+categories: [functions]
+menu:
+  docs:
+    parent: "functions"
+keywords: [collections]
+signature: ["PAGES | group KEY"]
+hugoversion: "0.49"
+---
+
+{{< code file="layouts/partials/groups.html" >}}
+{{ $new := .Site.RegularPages | first 10 | group "New" }}
+{{ $old := .Site.RegularPages | last 10 | group "Old" }}
+{{ $groups := slice $new $old }}
+{{ range $groups }}
+<h3>{{ .Key }}{{/* Prints "New", "Old" */}}</h3>
+<ul>
+    {{ range .Pages }}
+    <li>
+    <a href="{{ .Permalink }}">{{ .Title }}</a>
+    <div class="meta">{{ .Date.Format "Mon, Jan 2, 2006" }}</div>
+    </li>
+    {{ end }}
+</ul>
+{{ end }}
+{{< /code >}}
+
+
+
+The page group you get from `group` is of the same type you get from the built-in [group methods](/templates/lists#group-content) in Hugo. The above example can even be [paginated](/templates/pagination/#list-paginator-pages).
+
+
+
+