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/aspect-ratio.mdx

@@ -0,0 +1,27 @@
+---
+sidebar_position: 3
+---
+
+import Playground from '@site/src/components/Playground';
+
+# Aspect Ratio
+
+The `aspect ratio` property in Yoga has the following properties:
+
+- Accepts any floating point value > 0, the default is undefined.
+- Defined as the ratio between the `width` and the `height` of a node e.g. if a node has an aspect ratio of 2 then its `width` is twice the size of its `height`.
+- Respects the `min` and `max` dimensions of an item.
+- Has higher priority than `flex grow`
+- If `aspect ratio`, `width`, and `height` are set then the cross axis dimension is overridden.
+
+<Playground code={`<Layout config={{useWebDefaults: false}}>
+  <Node
+    style={{
+      width: 200,
+      height: 200,
+      padding: 10,
+    }}>
+    <Node style={{margin: 5, height: 50, aspectRatio: 1.0}} />
+    <Node style={{margin: 5, height: 50, aspectRatio: 1.5}} />
+  </Node>
+</Layout>`} />