docs: ExtensionManager class docs

docs/latest/SUMMARY.md

@@ -27,6 +27,7 @@
     - [Toolbar](extensions/modules/toolbar.md)
     - [Viewport](extensions/modules/viewport.md)
   - [Contexts](extensions/index.md#contexts)
+  - [ExtensionManager](extensions/index.md#extension-manager)
   - [OHIF Maintained](extensions/index.md#maintained-extensions)
 - [Services](services/index.md)
   - [Default](services/default/index.md)

docs/latest/extensions/index.md

@@ -8,6 +8,7 @@
   - [Modules](#modules)
   - [Contexts](#contexts)
 - [Consuming Extensions](#consuming-extensions)
+  - [Extension Manager](#extension-manager)
 - [Maintained Extensions](#maintained-extensions)
 
 ## Overview
@@ -188,6 +189,33 @@ with current theming, configuration, extension, and services support.
 If you're not sure how to achieve your goals with the extensibility available
 today, create a GitHub issue!
 
+### `ExtensionManager`
+
+The `ExtensionManager` is a class made available to us via the `@ohif/core`
+project (platform/core). Our application instantiates a single instance of it,
+and provides a `ServicesManager` and `CommandsManager`.
+
+```js
+const commandsManager = new CommandsManager();
+const servicesManager = new ServicesManager();
+const extensionManager = new ExtensionManager({
+  commandsManager,
+  servicesManager,
+});
+```
+
+The `ExtensionManager` only has a few public members:
+
+- `registerExtension` - Registers a single extension
+- `registerExtensions` - Registers an array of extensions
+- `modules` - An object containing registered extensions by `MODULE_TYPE`
+
+During registration, lifecycle hooks and modules have access to
+`ExtensionManager`'s `ServicesManager` and `CommandsManager`.
+
+Our `@ohif/viewer` uses the `modules` member to access registered extensions at
+appropriate places in our application.
+
 ## Maintained Extensions
 
 A small number of powerful extensions for popular use cases are maintained by