[ACA-1889] Split Extending section into separate files (#784)

* [ACA-1889] Split Extending section into separate files

* [ACA-1889] Split Extending section into separate files

* [ACA-1889] Added recent changes and then deleted extending.md

* [ACA-1889] Added Tutorials section

docs/extending/registration.md

@@ -0,0 +1,53 @@
+---
+---
+
+# Registration
+
+You can use `ExtensionService` to register custom components, authentication guards,
+rule evaluators, etc.
+
+It is recommended to register custom content from within the module constructor.
+In that case all plugins will be available right after the main application component is ready.
+
+Update the main application module `app.module.ts`, or create your own module,
+and use the following snippet to register custom content:
+
+```ts
+import { ExtensionsModule, ExtensionService } from '@alfresco/adf-extensions';
+
+@NgModule({
+    imports: [ ExtensionsModule.forChild() ]
+    declarations: [ MyComponent1, MyLayout ],
+    entryComponents: [ MyComponent1, MyLayout ]
+})
+export class MyExtensionModule {
+
+    constructor(extensions: ExtensionService) {
+        extensions.setComponents({
+            'plugin1.components.my': MyComponent1,
+            'plugin1.layouts.my': MyLayout
+        });
+
+        extensions.setAuthGuards({
+            'plugin.auth': MyAuthGuard
+        });
+
+        extensions.setEvaluators({
+            'plugin1.rules.custom1': MyCustom1Evaluator,
+            'plugin1.rules.custom2': MyCustom2Evaluator
+        });
+    }
+
+}
+```
+
+Use `ExtensionsModule.forChild()` when importing into the child modules,
+and `ExtensionsModule.forRoot()` for the main application module.
+
+<p class="warning">
+According to Angular rules, all components that are created dynamically at runtime
+need to be registered within the `entryComponents` section of the NgModule.
+</p>
+
+The Registration API is not limited to the custom content only.
+You can replace any existing entries by replacing the values from your module.