docs(button-toggle): add appearance docs and example (#13416)

As discussed in #12443, these changes add some docs and a live example for the button toggle `appearance`.

src/lib/button-toggle/button-toggle.md

@@ -1,5 +1,5 @@
-`<mat-button-toggle>` are on/off toggles with the appearance of a button. These toggles can be 
-configured to behave as either radio-buttons or checkboxes. While they can be standalone, they are 
+`<mat-button-toggle>` are on/off toggles with the appearance of a button. These toggles can be
+configured to behave as either radio-buttons or checkboxes. While they can be standalone, they are
 typically part of a `mat-button-toggle-group`.
 
 
@@ -7,16 +7,24 @@ typically part of a `mat-button-toggle-group`.
 
 ### Exclusive selection vs. multiple selection
 By default, `mat-button-toggle-group` acts like a radio-button group- only one item can be selected.
-In this mode, the `value` of the `mat-button-toggle-group` will reflect the value of the selected 
-button and `ngModel` is supported. 
+In this mode, the `value` of the `mat-button-toggle-group` will reflect the value of the selected
+button and `ngModel` is supported.
 
 Adding the `multiple` attribute allows multiple items to be selected (checkbox behavior). In this
-mode the values of the the toggles are not used, the `mat-button-toggle-group` does not have a value, 
+mode the values of the the toggles are not used, the `mat-button-toggle-group` does not have a value,
 and `ngModel` is not supported.
 
+### Appearance
+By default, the appearance of `mat-button-toggle-group` and `mat-button-toggle` will follow the
+latest Material Design guidelines. If you want to, you can switch back to the appearance that was
+following the previous Material Design spec by using the `appearance` input. The `appearance` can
+be configured globally using the `MAT_BUTTON_TOGGLE_DEFAULT_OPTIONS` injection token.
+
+<!-- example(button-toggle-appearance) -->
+
 ### Accessibility
 The button-toggles will present themselves as either checkboxes or radio-buttons based on the
-presence of the `multiple` attribute. 
+presence of the `multiple` attribute.
 
 For button toggles containing only icons, each button toggle should be given a meaningful label via
 `aria-label` or `aria-labelledby`.

src/material-examples/button-toggle-appearance/button-toggle-appearance-example.css

@@ -0,0 +1 @@
+/** No CSS for this example */

src/material-examples/button-toggle-appearance/button-toggle-appearance-example.html

@@ -0,0 +1,17 @@
+<p>
+  Default appearance:
+  <mat-button-toggle-group name="fontStyle" aria-label="Font Style">
+    <mat-button-toggle value="bold">Bold</mat-button-toggle>
+    <mat-button-toggle value="italic">Italic</mat-button-toggle>
+    <mat-button-toggle value="underline">Underline</mat-button-toggle>
+  </mat-button-toggle-group>
+</p>
+
+<p>
+  Legacy appearance:
+  <mat-button-toggle-group appearance="legacy" name="fontStyle" aria-label="Font Style">
+    <mat-button-toggle value="bold">Bold</mat-button-toggle>
+    <mat-button-toggle value="italic">Italic</mat-button-toggle>
+    <mat-button-toggle value="underline">Underline</mat-button-toggle>
+  </mat-button-toggle-group>
+</p>

src/material-examples/button-toggle-appearance/button-toggle-appearance-example.ts

@@ -0,0 +1,11 @@
+import {Component} from '@angular/core';
+
+/**
+ * @title Button toggle appearance
+ */
+@Component({
+  selector: 'button-toggle-appearance-example',
+  templateUrl: 'button-toggle-appearance-example.html',
+  styleUrls: ['button-toggle-appearance-example.css'],
+})
+export class ButtonToggleAppearanceExample {}