[Icon] Point to the public documentation (#2452)

* [Icon] Point to the public documentation

The Shopify/polaris-icons repository is now private, meaning non-Shopify employees would get a 404 by clicking on the link pointing to https://github.com/Shopify/polaris-icons/tree/master/packages/polaris-icons.

* Update UNRELEASED.md

UNRELEASED.md

@@ -11,6 +11,8 @@
 
 ### Documentation
 
+- Changed a link to the Polaris icons documentation so it would point to npm (a public resource) rather than the `Shopify/polaris-icons` repository (which is now private) ([#2452](https://github.com/Shopify/polaris-react/pull/2452))
+
 ### Development workflow
 
 ### Dependency upgrades

src/components/Icon/README.md

@@ -103,8 +103,6 @@ If the icon appears without text, then use the `accessibilityLabel` prop to give
 
 ## Related guidelines
 
-- To learn about implementing Polaris icons with [Polaris React](https://github.com/Shopify/polaris-react) in your projects, see the [`@shopify/polaris-icons` documentation](https://github.com/Shopify/polaris-icons/tree/master/packages/polaris-icons)
-
+- To learn about implementing Polaris icons with [Polaris React](https://github.com/Shopify/polaris-react) in your projects, see the [`@shopify/polaris-icons` documentation](https://www.npmjs.com/package/@shopify/polaris-icons)
 - To learn about the best practices for designing and using icons in your projects, see the [icon design guidelines](https://polaris.shopify.com/design/icons)
-
 - To learn how to name icons, see the [icon naming guidelines](https://polaris.shopify.com/content/naming#section-icons)