Add documentation for concat filter

Author: Adam Hollett

filters/concat.md

@@ -0,0 +1,58 @@
+---
+title: concat
+description: Liquid filter that concatenates arrays.
+---
+
+Concatenates (joins together) multiple arrays. The resulting array contains all the items from the input arrays.
+
+<p class="code-label">Input</p>
+{% raw %}
+```liquid
+{% assign fruits = "apples, oranges, peaches" | split: ", " %}
+{% assign vegetables = "carrots, turnips, potatoes" | split: ", " %}
+
+{% assign everything = fruits | concat: vegetables %}
+
+{% for item in everything %}
+- {{ item }}
+{% endfor %}
+```
+{% endraw %}
+
+<p class="code-label">Output</p>
+```text
+- apples
+- oranges
+- peaches
+- carrots
+- turnips
+- potatoes
+```
+
+You can string together `concat` filters to join more than two arrays:
+
+<p class="code-label">Input</p>
+{% raw %}
+```liquid
+{% assign furniture = "chairs, tables, shelves" | split: ", " %}
+
+{% assign everything = fruits | concat: vegetables | concat: furniture %}
+
+{% for item in everything %}
+- {{ item }}
+{% endfor %}
+```
+{% endraw %}
+
+<p class="code-label">Output</p>
+```text
+- apples
+- oranges
+- peaches
+- carrots
+- turnips
+- potatoes
+- chairs
+- tables
+- shelves
+```