[Doc] Add views layout best practices

Author: pkasting@chromium.org
Examples: tluk@chromium.org
Markdown: weili@chromium.org
Bug: none
Change-Id: Ibb81c34929b33479124d49aa2654468fc89564bc
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2224604
Commit-Queue: Wei Li <weili@chromium.org>
Reviewed-by: Peter Kasting <pkasting@chromium.org>
Cr-Commit-Position: refs/heads/master@{#774840}

docs/ui/learn/index.md

@@ -3,6 +3,7 @@
 # Best Practices
 
 * [Colors](bestpractices/colors.md): How to work with Chromium colors.
+* [Layout](bestpractices/layout.md): How to use Views layout.
 * [Ownership](bestpractices/ownership.md): How to manage Views object lifetimes.
 
 # Architecture Overview