docs: begin breaking services docs into bite sized pages

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

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+# UI Modal Service
+
+Modals have similar characteristics to that of Dialogs, but are often larger,
+and only allow for a single instance to be viewable at once. They also tend to
+be centered, and not draggable. They're commonly used when:
+
+- We need to grab the user's attention
+- We need user input
+- We need to show additional information
+
+If you're curious about the DOs and DON'Ts of dialogs and modals, check out this
+article: ["Best Practices for Modals / Overlays / Dialog Windows"][ux-article]
+
+<div style="text-align: center;">
+  <a href="/assets/img/modal-example.gif">
+    <img src="/assets/img/modal-example.gif" alt="UI Modal Service Example" style="margin: 0 auto; max-width: 500px;" />
+  </a>
+  <div><i>GIF showing successful call of UIModalService 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 open modal                  |
+| `show()`   | Shows the provided content in a modal |
+
+## Implementations
+
+| Implementation                     | Consumer                      |
+| ---------------------------------- | ----------------------------- |
+| [Modal Provider][modal-provider]\* | [OHIFModal][modal-consumer]\* |
+
+`*` - 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/UIModalService/index.js
+[modal-provider]: https://github.com/OHIF/Viewers/blob/master/platform/ui/src/contextProviders/ModalProvider.js
+[modal-consumer]: https://github.com/OHIF/Viewers/tree/master/platform/ui/src/components/ohifModal
+[ux-article]: https://uxplanet.org/best-practices-for-modals-overlays-dialog-windows-c00c66cddd8c
+<!-- prettier-ignore-end -->