Add doc on desktop input event routing

A comment-friendly doc version is available at
http://doc/1jBT-aqlwb4I7_OH6xC4yX5sn8zlZTOXbfYiU4coG0yQ

Bug: None
Change-Id: I135f0ba144c437af6bd366625e8cc989c246d7a6
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2571803Reviewed-by: Robert Liao <robliao@chromium.org>
Commit-Queue: Robert Liao <robliao@chromium.org>
Cr-Commit-Position: refs/heads/master@{#833769}

docs/ui/index.md

@@ -20,6 +20,7 @@ Details on Chrome UI.
 * [Platform Styling](/docs/ui/views/platform_style.md)
 * [Product Excellence](/docs/ui/product_excellence/index.md)
 * [UI Devtools](/docs/ui/ui_devtools/index.md)
+* [Input Event Routing](/docs/ui/input_event/index.md)
 
 Archival Documentation on Chrome UI.
 * [Aura](/docs/ui/aura/index.md)

docs/ui/learn/index.md

@@ -11,6 +11,7 @@
 * [Views](/docs/ui/views/overview.md)
 * [Product Excellence](/docs/ui/product_excellence/index.md)
 * [UI Devtools](/docs/ui/ui_devtools/index.md)
+* [Input Event Routing](/docs/ui/input_event/index.md)
 
 # Archival Documentation on Chrome UI.