Add justify content documentation (#1598)

Summary:
Pull Request resolved: https://github.com/facebook/yoga/pull/1598

tisa

Reviewed By: NickGerleman

Differential Revision: D54778011

fbshipit-source-id: d1a38a88da3e8534577f87ea410bd8d8b3ecc6be

website-next/docs/examples/aspect-ratio.mdx

@@ -19,7 +19,7 @@ The `aspect ratio` property in Yoga has the following properties:
     style={{
       width: 200,
       height: 200,
-      padding: 20,
+      padding: 10,
     }}>
     <Node style={{margin: 5, height: 50, aspectRatio: 1.0}} />
     <Node style={{margin: 5, height: 50, aspectRatio: 1.5}} />

website-next/docs/examples/display.mdx

@@ -17,7 +17,7 @@ Display controls which layout specification to follow.
     style={{
       width: 200,
       height: 200,
-      padding: 20,
+      padding: 10,
     }}>
     <Node style={{margin: 5, height: 50, display: 'flex'}} />
     <Node style={{margin: 5, height: 50, display: 'none'}} />

website-next/docs/examples/flex-direction.mdx

@@ -28,7 +28,7 @@ the next line will start to the left first item on the bottom of the container.
     style={{
       width: 200,
       height: 200,
-      padding: 20,
+      padding: 10,
       flexDirection: 'column'
     }}>
     <Node style={{margin: 5, height: 50, width: 50}} />

website-next/docs/examples/justify-content.mdx

@@ -2,4 +2,42 @@
 sidebar_position: 9
 ---
 
+import Playground from '@site/src/components/Playground';
+
 # Justify Content
+
+Justify content describes how to align children within the main axis of their container.
+For example, you can use this property to center a child horizontally within a container
+with `flex direction` set to `row` or vertically within a container with `flex direction`
+set to `column`.
+
+**Flex start (default)**: Align children of a container to the start of the container's main axis.
+
+**Flex end**: Align children of a container to the end of the container's main axis.
+
+**Center**: Align children of a container in the center of the container's main axis.
+
+**Space between**: Evenly space of children across the container's main axis, distributing
+remaining space between the children.
+
+**Space around**: Evenly space of children across the container's main axis, distributing
+remaining space around the children. Compared to `space between` using
+`space around` will result in space being distributed to the beginning of
+the first child and end of the last child.
+
+**Space evenly**: Evenly distributed within the alignment container along the main axis.
+The spacing between each pair of adjacent items, the main-start edge and the first item,
+and the main-end edge and the last item, are all exactly the same.
+
+<Playground code={`<Layout config={{useWebDefaults: false}}>
+  <Node
+    style={{
+      width: 200,
+      height: 200,
+      padding: 10,
+      justifyContent: 'flex-start'
+    }}>
+    <Node style={{margin: 5, height: 50, width: 50}} />
+    <Node style={{margin: 5, height: 50, width: 50}} />
+  </Node>
+</Layout>`} />