Add doc for advanced metadata definitions and usages

This doc explains a few advanced metadata and property definitions and
usages.

Original author: [email protected]

Bug: none
Change-Id: Ic7c8af46bdfe6977c31e9f08e955304b3e22e127
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2737096
Reviewed-by: Allen Bauer <[email protected]>
Commit-Queue: Wei Li <[email protected]>
Cr-Commit-Position: refs/heads/master@{#860013}
diff --git a/docs/ui/learn/index.md b/docs/ui/learn/index.md
index 61069b64..206cfa5a 100644
--- a/docs/ui/learn/index.md
+++ b/docs/ui/learn/index.md
@@ -12,6 +12,7 @@
 * [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)
+* [Metadata and Properties](/docs/ui/views/metadata_properties.md)
 
 # Archival Documentation on Chrome UI.