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/advanced/_category_.json

@@ -0,0 +1,4 @@
+{
+  "label": "Advanced",
+  "position": 3
+}

website-next/docs/advanced/caching.md

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+---
+sidebar_position: 1
+---
+
+# Caching and incremental layout
+
+Discuss invalidation model, hasNewLayout, etc.

website-next/docs/advanced/errata.md

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+---
+sidebar_position: 4
+---
+
+# Errata and layout conformance
+
+Talk about errata

website-next/docs/advanced/external-layout-systems.md

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+---
+sidebar_position: 2
+---
+
+# Integrating with external layout systems
+
+Talk about measure functions et all.

website-next/docs/advanced/pixel-grid.md

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+---
+sidebar_position: 3
+---
+
+# Pixel grid alignment
+
+Talk about pointScaleFactor and alignment to pixel grid.