docs: begin breaking services docs into bite sized pages

docs/latest/services/ui-notification-service.md

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+# UI Notification Service
+
+Notifications can be annoying and disruptive. They can also deliver timely
+helpful information, or expedite the user's workflow. Here is some high level
+guidance on when and how to use them:
+
+- Notifications should be non-interfering (timely, relevant, important)
+- We should only show small/brief notifications
+- Notifications should be contextual to current behavior/actions
+- Notifications can serve warnings (acting as a confirmation)
+
+If you're curious about the DOs and DON'Ts of notifications, check out this
+article: ["How To Design Notifications For Better UX"][ux-article]
+
+<div style="text-align: center;">
+  <a href="/assets/img/notification-example.gif">
+    <img src="/assets/img/notification-example.gif" alt="UI Notification Service Example" style="margin: 0 auto; max-width: 500px;" />
+  </a>
+  <div><i>GIF showing successful call of UINotificationService from an extension.</i></div>
+</div>
+
+## Interface
+
+For a more detailed look on the options and return values each of these methods
+is expected to support, [check out it's interface in `@ohif/core`][interface]
+
+| API Member | Description                             |
+| ---------- | --------------------------------------- |
+| `hide()`   | Hides the specified notification        |
+| `show()`   | Creates and displays a new notification |
+
+## Implementations
+
+| Implementation                           | Consumer                                  |
+| ---------------------------------------- | ----------------------------------------- |
+| [Snackbar Provider][snackbar-provider]\* | [SnackbarContainer][snackbar-container]\* |
+
+`*` - Denotes maintained by OHIF
+
+> 3rd Party implementers may be added to this table via pull requests.
+
+<!--
+  LINKS
+-->
+
+<!-- prettier-ignore-start -->
+[interface]: https://github.com/OHIF/Viewers/blob/master/platform/core/src/services/UINotificationService/index.js
+[snackbar-provider]: https://github.com/OHIF/Viewers/blob/master/platform/ui/src/contextProviders/SnackbarProvider.js
+[snackbar-container]: https://github.com/OHIF/Viewers/blob/master/platform/ui/src/components/snackbar/SnackbarContainer.js
+[ux-article]: https://uxplanet.org/how-to-design-notifications-for-better-ux-6fb0711be54d
+<!-- prettier-ignore-end -->