docs: extensions (#1258)

* docs: process docs to include UX Stories requirement

* docs: include note regarding different environments

* Services init

* Remove unused canny logos

* docs: add ModalService diagram

* docs: GIF of notification

* docs: add ui-services page

* docs: simplify ui services call out in the general services docs

* docs: tips and tricks for UI services

* docs: moar pages

* docs: dialog gif

* docs: services in summary/sidebar

* docs: gif examples at top of dialog and notification pages

* docs: add UIModal gif

* docs: details for Dialog Service

* docs: include usage information for ui modal service

* docs: detailed information about our UI Notification Service

* docs: services diagram

* docs: remove unused links

* docs: services example and image

* docs: stubbing extension pages

* docs: UX Stories --> User Cases

* chore: eslint fix

* docs: extensions overview + diagram

* docs: update extensions in sidebar

* docs: extension skeleton and registration info

* docs: copy of maintained extensions table partial in the extensions folder

* docs: swap out old extensions structure for new

docs/latest/extensions/modules/viewport.md

@@ -0,0 +1,29 @@
+# Module: Viewport
+
+An extension can register a Viewport Module by providing a `getViewportModule()`
+method that returns a React Component. The React component will receive the
+following props:
+
+```js
+children: PropTypes.arrayOf(PropTypes.element)
+studies: PropTypes.object,
+displaySet: PropTypes.object,
+viewportData: PropTypes.object, // { studies, displaySet }
+viewportIndex: PropTypes.number,
+children: PropTypes.node,
+customProps: PropTypes.object
+```
+
+Viewport components are managed by the `ViewportGrid` Component. Which Viewport
+component is used depends on:
+
+- The Layout Configuration
+- Registered SopClassHandlers
+- The SopClassUID for visible/selected datasets
+
+![Cornerstone Viewport](../../assets/img/extensions-viewport.png)
+
+<center><i>An example of three Viewports</i></center>
+
+For a complete example implementation,
+[check out the OHIFCornerstoneViewport](https://github.com/OHIF/Viewers/blob/master/extensions/cornerstone/src/OHIFCornerstoneViewport.js).