docs: improved code smells (#27185)

* docs: improved code smells

* alt description updated

(cherry picked from commit fee78c5)

Author: aanchal88

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

@@ -44,7 +44,7 @@
   </mat-card>
 
   <mat-card [appearance]="appearance">
-    <img mat-card-image src="https://material.angularjs.org/latest/img/washedout.png">
+    <img mat-card-image src="https://material.angularjs.org/latest/img/washedout.png" alt="Para Washed Out">
     <mat-card-header>
       <mat-card-title>Content Title</mat-card-title>
     </mat-card-header>
@@ -63,7 +63,7 @@
       <mat-card-title>Header title</mat-card-title>
       <mat-card-subtitle>Header subtitle</mat-card-subtitle>
     </mat-card-header>
-    <img mat-card-image src="https://material.angularjs.org/latest/img/washedout.png">
+    <img alt="Para Washed Out" mat-card-image src="https://material.angularjs.org/latest/img/washedout.png">
     <mat-card-content>
       <p>Here is some content</p>
     </mat-card-content>
@@ -140,7 +140,7 @@ <h2>Cards with media area</h2>
       <mat-card-title>Header title</mat-card-title>
       <mat-card-subtitle>Header subtitle</mat-card-subtitle>
     </mat-card-header>
-    <img mat-card-image src="https://material.angularjs.org/latest/img/washedout.png">
+    <img alt="Para Washed Out" mat-card-image src="https://material.angularjs.org/latest/img/washedout.png">
     <mat-card-content>
       <p>Here is some content</p>
     </mat-card-content>

src/material/bottom-sheet/bottom-sheet-container.ts

@@ -11,7 +11,6 @@ import {CdkDialogContainer, DialogConfig} from '@angular/cdk/dialog';
 import {FocusMonitor, FocusTrapFactory, InteractivityChecker} from '@angular/cdk/a11y';
 import {BreakpointObserver, Breakpoints} from '@angular/cdk/layout';
 import {OverlayRef} from '@angular/cdk/overlay';
-import {_getFocusedElementPierceShadowDom} from '@angular/cdk/platform';
 import {DOCUMENT} from '@angular/common';
 import {
   ChangeDetectionStrategy,