Add skeleton for documentation (#1589)

Summary:
Pull Request resolved: https://github.com/facebook/yoga/pull/1589

Adds a skeleton to the new website of documentation and such for us to fill out.

Reviewed By: joevilches

Differential Revision: D54567186

fbshipit-source-id: 700dfe0787fcb9806c3daa0675e838540119bbc0

website-next/docs/examples/_category_.json

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+{
+  "label": "Examples",
+  "position": 4
+}

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

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+---
+sidebar_position: 1
+---
+
+import Playground from '@site/src/components/Playground';
+
+# Align Content
+
+<Playground code={`<Layout config={{useWebDefaults: false}}>
+  <Node
+    style={{width: 200, height: 200, padding: 20}}
+  >
+    <Node style={{flex: 1}} />
+  </Node>
+</Layout>`} />

website-next/docs/examples/align-items-self.mdx

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+---
+sidebar_position: 2
+---
+
+# Align Items and Align Self

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

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+---
+sidebar_position: 3
+---
+
+# Aspect Ratio

website-next/docs/examples/display.mdx

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+---
+sidebar_position: 4
+---
+
+# Display

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

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+---
+sidebar_position: 5
+---
+
+# Flex Basis, Grow, and Shrink

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

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+---
+sidebar_position: 6
+---
+
+# Flex Direction

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

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+---
+sidebar_position: 7
+---
+
+# Flex Wrap

website-next/docs/examples/gap.mdx

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+---
+sidebar_position: 8
+---
+
+# Gap

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

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+---
+sidebar_position: 9
+---
+
+# Justify Content

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

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+---
+sidebar_position: 10
+---
+
+# Layout Direction

website-next/docs/examples/margin-padding-border.mdx

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+---
+sidebar_position: 11
+---
+
+# Margin, Padding, and Border

website-next/docs/examples/min-max-width-height.mdx

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+---
+sidebar_position: 12
+---
+
+# Min/Max Width and Height

website-next/docs/examples/position.mdx

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+---
+sidebar_position: 13
+---
+
+# Absolute/Relative/Static Positioning

website-next/docs/examples/width-height.mdx

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+---
+sidebar_position: 14
+---
+
+# Width and Height