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
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+# Extensions
+
+- [Overview](#overview)
+- [Concepts](#concepts)
+  - [Extension Skeleton](#extension-skeleton)
+  - [Registering an Extension](#registering-an-extension)
+  - [Lifecylce Hooks](#lifecycle-hooks)
+  - [Modules](#modules)
+  - [Contexts](#contexts)
+- [Consuming Extensions](#consuming-extensions)
+- [Maintained Extensions](#maintained-extensions)
+
+## Overview
+
+We use extensions to help us isolate and package groups of related features.
+Extensions provide functionality, ui components, and new behaviors.
+
+<div style="text-align: center;">
+  <a href="/assets/img/extensions-diagram.png">
+    <img src="/assets/img/extensions-diagram.png" alt="Extensions Diagram" style="margin: 0 auto; max-width: 500px;" />
+  </a>
+  <div><i>Diagram showing how extensions are configured and accessed.</i></div>
+</div>
+
+The `@ohif/viewer`'s application level configuration gives us the ability to add
+and configure extensions. When the application starts, extensions are registered
+with the `ExtensionManager`. Different portions of the `@ohif/viewer` project
+will use registered extensions to influence application behavior.
+
+Extensions allow us to:
+
+- Wrap and integrate functionality of 3rd party dependencies in a reusable way
+- Change how application data is mapped and transformed
+- Display a consistent/cohesive UI
+- Inject custom components to override built-in components
+
+Practical examples of extensions include:
+
+- A set of segmentation tools that build on top of the `cornerstone` viewport
+- Showing ML/AI report summaries for the selected study/series/image
+- Support for parsing DICOM structured reports and displaying them in a user
+  friendly way
+- [See our maintained extensions for more examples of what's possible](#maintained-extensions)
+
+## Concepts
+
+### Extension Skeleton
+
+An extension is a plain JavaScript object has an `id` property, and one or more
+"getModuleFunctions" and/or lifecycle hooks. You can read more about
+[lifecycle hooks](#lifecycle-hooks) and [modules](#modules) further down.
+
+```js
+// prettier-ignore
+export default {
+  /**
+   * Only required property. Should be a unique value across all extensions.
+   */
+  id: 'example-extension',
+
+  // Lifecyle
+  preRegistration() { /* */ },
+  // Modules
+  getCommandsModule() { /* */ },
+  getToolbarModule() { /* */ },
+  getPanelModule() { /* */ },
+  getSopClassHandler() { /* */ },
+  getViewportModule() { /* */ },
+}
+```
+
+### Registering an Extension
+
+There are two different ways to register and configure extensions. You can
+leverage one or both strategies. Which one(s) you choose depend on your
+application's requirements. Each [module](#modules) defined by the extension
+becomes available to the core application via the `ExtensionManager`.
+
+```js
+// prettier-ignore
+const config = {
+  extensions: [
+    MyFirstExtension,
+    [
+      MySecondExtension,
+      { /* MySecondExtensions Configuration */ },
+    ],
+  ];
+}
+```
+
+#### Runtime Extensions
+
+The `@ohif/viewer` uses a [configuration file](#) at startup. The schema for
+that file includes an `Extensions` key that supports an array of extensions to
+register.
+
+#### Bundled Extensions
+
+The `@ohif/viewer` works best when built as a "Progressive Web Application"
+(PWA). If you know the extensions your application will need, you can specify
+them at "build time" to leverage some advantaged afforded to us by modern
+tooling:
+
+- Code Splitting
+- Tree Shaking
+- Dependency deduplication
+
+You can update the list of bundled extensions by:
+
+1. Having your `@ohif/viewer` project depend on the extension
+2. Importing and adding it to the list of extensions in the
+   `<repo-root>/platform/src/index.js` entrypoint.
+
+### Lifecycle Hooks
+
+...
+
+### Modules
+
+There are a few different module types. Each module type allows us to extend the
+viewer in a different way, and provides a consistent API for us to do so. You
+can find a full list of the different types of modules
+[`in ohif-core`][module-types]. Information on each type of module, it's API,
+and how we determine when/where it should be used is included below.
+
+### Contexts
+
+...
+
+## Consuming Extensions
+
+...
+
+## Maintained Extensions
+
+A small number of powerful extensions for popular use cases are maintained by
+OHIF. They're co-located in the [`OHIF/Viewers`][viewers-repo] repository, in
+the top level [`extensions/`][ext-source] directory.
+
+{% include "./_maintained-extensions-table.md" %}
+
+<!--
+  LINKS
+-->
+
+<!-- prettier-ignore-start -->
+[viewers-repo]: https://github.com/OHIF/Viewers
+[ext-source]: https://github.com/OHIF/Viewers/tree/master/extensions
+[module-types]: https://github.com/OHIF/Viewers/blob/master/platform/core/src/extensions/MODULE_TYPES.js
+<!-- prettier-ignore-end -->