docs(cdk/menu): resolve accessibility issues with examples

Fixes the following accessibility issues with the CDK menu examples:
* Some menu triggers were incorrectly marked as `cdkMenuItem` which meant that they had `role="menuitem"` even though they weren't inside menus.
* All the menu examples had `outline: none` which made it difficult to track focus when navigating with the keyboard.

Author: crisbeto

src/components-examples/cdk/menu/cdk-menu-context/cdk-menu-context-example.css

@@ -10,7 +10,6 @@
 .example-menu-item {
   background-color: transparent;
   cursor: pointer;
-  outline: none;
   border: none;
 
   user-select: none;

src/components-examples/cdk/menu/cdk-menu-inline/cdk-menu-inline-example.css

@@ -10,7 +10,6 @@
 .example-menu-item {
   background-color: transparent;
   cursor: pointer;
-  outline: none;
   border: none;
 
   user-select: none;

src/components-examples/cdk/menu/cdk-menu-menubar/cdk-menu-menubar-example.css

@@ -1,6 +1,5 @@
 .example-menu-bar-item {
   cursor: pointer;
-  outline: none;
   border: none;
 
   user-select: none;
@@ -35,7 +34,6 @@
 .example-menu .example-menu-item {
   background-color: transparent;
   cursor: pointer;
-  outline: none;
   border: none;
 
   user-select: none;

src/components-examples/cdk/menu/cdk-menu-standalone-menu/cdk-menu-standalone-menu-example.css

@@ -11,7 +11,6 @@
 .example-standalone-item {
   background-color: transparent;
   cursor: pointer;
-  outline: none;
   border: none;
 
   user-select: none;

src/components-examples/cdk/menu/cdk-menu-standalone-menu/cdk-menu-standalone-menu-example.html

@@ -1,4 +1,4 @@
-<button cdkMenuItem [cdkMenuTriggerFor]="menu" class="example-standalone-item">Click me!</button>
+<button [cdkMenuTriggerFor]="menu" class="example-standalone-item">Click me!</button>
 
 <ng-template #menu>
   <div class="example-menu" cdkMenu>

src/components-examples/cdk/menu/cdk-menu-standalone-stateful-menu/cdk-menu-standalone-stateful-menu-example.css

@@ -20,7 +20,6 @@ hr {
 .example-standalone-item {
   background-color: transparent;
   cursor: pointer;
-  outline: none;
   border: none;
 
   user-select: none;

src/components-examples/cdk/menu/cdk-menu-standalone-stateful-menu/cdk-menu-standalone-stateful-menu-example.html

@@ -1,4 +1,4 @@
-<button cdkMenuItem [cdkMenuTriggerFor]="menu" class="example-standalone-item">Click me!</button>
+<button [cdkMenuTriggerFor]="menu" class="example-standalone-item">Click me!</button>
 
 <ng-template #menu>
   <div class="example-menu" cdkMenu>

src/dev-app/cdk-menu/cdk-menu-demo.html

@@ -160,9 +160,9 @@ <h4>Custom Context Menu Position (Centered on Cursor)</h4>
 
 <div class="example-menu-container">
   <h3>Radio items</h3>
-  <button cdkMenuItem [cdkMenuTriggerFor]="sizeMenu">Size menu (items directly in menu)</button>
-  <button cdkMenuItem [cdkMenuTriggerFor]="colorMenu">Color menu (items in menu group)</button>
-  <button cdkMenuItem [cdkMenuTriggerFor]="sizeAndColorMenu">Size & Color menu (items in directly in menu & in menu group)</button>
+  <button [cdkMenuTriggerFor]="sizeMenu">Size menu (items directly in menu)</button>
+  <button [cdkMenuTriggerFor]="colorMenu">Color menu (items in menu group)</button>
+  <button [cdkMenuTriggerFor]="sizeAndColorMenu">Size & Color menu (items in directly in menu & in menu group)</button>
 
   <ng-template #sizeMenu>
     <div class="example-menu" cdkMenu>