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/getting-started/_category_.json

@@ -0,0 +1,4 @@
+{
+  "label": "Getting Started",
+  "position": 2
+}

website-next/docs/getting-started/configuring-yoga.md

@@ -0,0 +1,7 @@
+---
+sidebar_position: 2
+---
+
+# Configuring Yoga
+
+Talk about YGConfig model. Per subtree configuration. UseWebDefaults. Link to advanced topics.

website-next/docs/getting-started/laying-out-a-tree.md

@@ -0,0 +1,7 @@
+---
+sidebar_position: 1
+---
+
+# Laying out a Yoga tree
+
+Introduction which covers basic APIs for creating then laying out a Yoga tree, then reading the layout output.

website-next/docs/getting-started/styling.md

@@ -0,0 +1,7 @@
+---
+sidebar_position: 3
+---
+
+# Styling
+
+Talk about setting Yoga styles, units, logical edges, any quirks/invalidation, style defaults.