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/align-content.mdx

@@ -0,0 +1,42 @@
+---
+sidebar_position: 1
+---
+
+import Playground from '@site/src/components/Playground';
+
+# Align Content
+
+Align content defines the distribution of lines along the cross-axis. This only
+has effect when items are wrapped to multiple lines using [`flex wrap`](/docs/styling/flex-wrap).
+
+**Flex start (default)**: Align wrapped lines to the start of the container's cross axis.
+
+**Flex end**: Align wrapped lines to the end of the container's cross axis.
+
+**Stretch**: Stretch wrapped lines to match the `height` of the container's cross axis.
+
+**Center**: Align wrapped lines in the center of the container's cross axis.
+
+**Space between**: Evenly space wrapped lines across the container's main axis, distributing
+remaining space between the lines.
+
+**Space around**: Evenly space wrapped lines across the container's main axis, distributing
+remaining space around the lines. Compared to `space between` using
+`space around` will result in space being distributed to the begining of
+the first lines and end of the last line.
+
+<Playground code={`<Layout config={{useWebDefaults: false}}>
+  <Node
+    style={{
+      width: 250,
+      height: 250,
+      padding: 10,
+      alignContent: 'flex-start',
+      flexWrap: 'wrap'
+    }}>
+    <Node style={{margin: 5, height: 50, width: 50}} />
+    <Node style={{margin: 5, height: 50, width: 50}} />
+    <Node style={{margin: 5, height: 50, width: 50}} />
+    <Node style={{margin: 5, height: 50, width: 50}} />
+  </Node>
+</Layout>`} />