Modularize and document public API (#1449)

Summary:
X-link: https://github.com/facebook/react-native/pull/41317

Pull Request resolved: https://github.com/facebook/yoga/pull/1449

This aims to clean up the public Yoga C API, by:
1. Documenting public YGNode, YGValue, YGConfig APIs
2. Splitting APIs for specific objects into different header files (because Yoga.h was big enough without documentation)
3. Reordering headers and definitions for consistent grouping

Changelog: [Internal]

Reviewed By: joevilches

Differential Revision: D50963424

fbshipit-source-id: 45124b7370256fc63aefd6d5b7641466e9a79d3b

yoga/module.modulemap

@@ -7,8 +7,13 @@
 
 module yoga [system] {
     module core {
+        header "YGConfig.h"
         header "YGEnums.h"
         header "YGMacros.h"
+        header "YGNode.h"
+        header "YGNodeLayout.h"
+        header "YGNodeStye.h"
+        header "YGPixelGrid.h"
         header "YGValue.h"
         header "Yoga.h"
         export *