docs(examples/cdk/overlay): add overlay example component to dev-app

Author: andy9775

src/dev-app/overlay/overlay-demo-module.ts

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+/**
+ * @license
+ * Copyright Google LLC All Rights Reserved.
+ *
+ * Use of this source code is governed by an MIT-style license that can be
+ * found in the LICENSE file at https://angular.io/license
+ */
+import {NgModule} from '@angular/core';
+import {RouterModule} from '@angular/router';
+import {OverlayModule} from '@angular/cdk/overlay';
+import {CdkOverlayExamplesModule} from '@angular/components-examples/cdk/overlay';
+
+import {OverlayDemo} from './overlay-demo';
+
+@NgModule({
+  imports: [
+    OverlayModule,
+    CdkOverlayExamplesModule,
+    RouterModule.forChild([{path: '', component: OverlayDemo}]),
+  ],
+  declarations: [OverlayDemo],
+})
+export class OverlayDemoModule {}

src/dev-app/overlay/overlay-demo.html

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+<h3>Basic overlay</h3>
+<cdk-overlay-basic-example></cdk-overlay-basic-example>

src/dev-app/overlay/overlay-demo.ts

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+/**
+ * @license
+ * Copyright Google LLC All Rights Reserved.
+ *
+ * Use of this source code is governed by an MIT-style license that can be
+ * found in the LICENSE file at https://angular.io/license
+ */
+import {Component} from '@angular/core';
+
+@Component({
+  templateUrl: 'overlay-demo.html',
+})
+export class OverlayDemo {}