Add flex basis grow shrink documentation (#1607)

Summary:
Pull Request resolved: #1607

tsia

Reviewed By: yungsters

Differential Revision: D54783674

fbshipit-source-id: 95530db0b129d8f0fb3e677430b9bfbf0662f590

website-next/docs/examples/flex-basis-grow-shrink.mdx

@@ -2,4 +2,64 @@
 sidebar_position: 5
 ---
 
+import Playground from '@site/src/components/Playground';
+
 # Flex Basis, Grow, and Shrink
+
+**Flex basis**: is an axis-independent way of providing the default size of an item
+along the main axis. Setting the flex basis of a child is similar to setting the `width` of that
+child if its parent is a container with `flex direction: row` or setting the `height` of a child
+if its parent is a container with `flex direction: column`. The flex basis of an item is the
+default size of that item, the size of the item before any flex grow and flex shrink
+calculations are performed.
+
+<Playground code={`<Layout config={{useWebDefaults: false}}>
+  <Node
+    style={{
+      width: 200,
+      height: 200,
+      padding: 10,
+    }}>
+    <Node style={{margin: 5, flexBasis: 50}} />
+  </Node>
+</Layout>`} />
+
+**Flex grow**: describes how any space within a container should be distributed
+among its children along the main axis. After laying out its children, a container will
+distribute any remaining space according to the flex grow values specified by its children.
+
+Flex grow accepts any floating point value >= 0, with 0 being the default value.
+A container will distribute any remaining space among its children weighted by the child’s flex grow value.
+
+<Playground code={`<Layout config={{useWebDefaults: false}}>
+  <Node
+    style={{
+      width: 200,
+      height: 200,
+      padding: 10,
+    }}>
+    <Node style={{margin: 5, flexGrow: 0.25}} />
+    <Node style={{margin: 5, flexGrow: 0.75}} />
+  </Node>
+</Layout>`} />
+
+**Flex shrink**: describes how to shrink children along the main axis in the
+case that the total size of the children overflow the size of the container on the main axis.
+flex shrink is very similar to flex grow and can be thought of in the same way if
+any overflowing size is considered to be negative remaining space.
+These two properties also work well together by allowing children to grow and shrink as needed.
+
+Flex shrink accepts any floating point value >= 0, with 1 being the default value.
+A container will shrink its children weighted by the child’s flex shrink value.
+
+<Playground code={`<Layout config={{useWebDefaults: false}}>
+  <Node
+    style={{
+      width: 200,
+      height: 200,
+      padding: 10,
+    }}>
+    <Node style={{margin: 5, flexShrink: 5, height: 150}} />
+    <Node style={{margin: 5, flexShrink: 10, height: 150}} />
+  </Node>
+</Layout>`} />