Fill out "About Yoga", "Laying out a Yoga tree" , and "Styling" (#1591)

Summary:
Pull Request resolved: https://github.com/facebook/yoga/pull/1591

Adds initial documentation to these sections.

Reviewed By: joevilches

Differential Revision: D54708141

fbshipit-source-id: b2c1ac20573840833a3e5fde8c7b53f2770cecbd

website-next/docs/styling/layout-direction.mdx

@@ -0,0 +1,34 @@
+---
+sidebar_position: 10
+---
+
+import Playground from '@site/src/components/Playground';
+
+# Layout Direction
+
+Layout direction specifies the direction in which children and text
+in a hierarchy should be laid out. Layout direction also effects what
+edge `start` and `end` refer to. By default Yoga lays out with `LTR`
+layout direction. In this mode `start` refers to `left` and `end`
+refers to `right`. When localizing your apps for markets with RTL languages
+you should customize this by either by passing a direction
+to the `CalculateLayout` call or by setting the direction on the root node.
+
+**LTR (default)**: Text and children and laid out from left to right. Styles applied
+the start of an element are applied on the left side.
+
+**RTL**: Text and children and laid out from right to left. Styles applied the
+start of an element are applied on the right side.
+
+<Playground code={`<Layout config={{useWebDefaults: false}}>
+  <Node
+    style={{
+      width: 200,
+      height: 200,
+      padding: 10,
+      direction: 'ltr'
+    }}>
+    <Node style={{margin: 5, height: 50, width: 50}} />
+    <Node style={{margin: 5, height: 50, width: 50}} />
+  </Node>
+</Layout>`} />