docs: adds new button examples (#11975)

Author: FrederikSchlemmer

src/material-examples/button-types/button-types-example.html

@@ -28,6 +28,16 @@ <h3>Stroked Buttons</h3>
   <a mat-stroked-button routerLink=".">Link</a>
 </div>
 
+<h3>Flat Buttons</h3>
+<div class="button-row">
+  <button mat-flat-button>Basic</button>
+  <button mat-flat-button color="primary">Primary</button>
+  <button mat-flat-button color="accent">Accent</button>
+  <button mat-flat-button color="warn">Warn</button>
+  <button mat-flat-button disabled>Disabled</button>
+  <a mat-flat-button routerLink=".">Link</a>
+</div>
+
 <h3>Icon Buttons</h3>
 <div class="button-row">
   <button mat-icon-button>
@@ -71,4 +81,4 @@ <h3>Mini Fab Buttons</h3>
     <mat-icon aria-label="Example icon-button with a heart icon">favorite</mat-icon>
   </button>
   <a mat-mini-fab routerLink=".">Link</a>
-</div>
+</div>