Add docs on platform-specific button order and string casing

Bug: none
Change-Id: I90367acee185648eac442ea7b317672c464d3b72
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2437693
Commit-Queue: Keren Zhu <[email protected]>
Reviewed-by: Elly Fong-Jones <[email protected]>
Cr-Commit-Position: refs/heads/master@{#812222}
diff --git a/docs/ui/index.md b/docs/ui/index.md
index 41ff3c1..749ab49 100644
--- a/docs/ui/index.md
+++ b/docs/ui/index.md
@@ -17,6 +17,7 @@
 
 * [Best Practices](/docs/ui/learn/index.md#best-practices)
 * [Views](/docs/ui/views/overview.md)
+* [Platform Styling](/docs/ui/views/platform_style.md)
 * [Product Excellence](/docs/ui/product_excellence/index.md)
 * [UI Devtools](/docs/ui/ui_devtools/index.md)