fix(docs-infra): use up-to-date URLs in "Contribute" links (and replace deprecated `md-button`) (#40944)

This commit replaces some outdated URLs in "Contribute" links (on the
"Contribute" page):
- `github.com/angular/material2` --> `github.com/angular/components`
- `github.com/angular/angularfire2` --> `github.com/angular/angularfire`

It also replaces the deprecated `md-button` attribute with `mat-button`.

PR Close #40944

aio/content/marketing/contribute.html

@@ -14,7 +14,9 @@
       Angular is a next generation mobile and desktop application development platform.
 
     </div>
-    <a href="https://github.com/angular/angular/blob/master/CONTRIBUTING.md" class="button button-blue" md-button>Contribute</a>
+    <a href="https://github.com/angular/angular/blob/master/CONTRIBUTING.md" class="button button-blue" mat-button>
+      Contribute
+    </a>
   </div>
 
   <div class="card-section">
@@ -25,7 +27,9 @@
 
     </div>
 
-    <a href="https://github.com/angular/angular-cli/blob/master/CONTRIBUTING.md" class="button button-blue" md-button>Contribute</a>
+    <a href="https://github.com/angular/angular-cli/blob/master/CONTRIBUTING.md" class="button button-blue" mat-button>
+      Contribute
+    </a>
   </div>
 
   <div class="card-section">
@@ -37,7 +41,9 @@
 
     </div>
 
-    <a href="https://github.com/angular/material2/blob/master/CONTRIBUTING.md" class="button button-blue" md-button>Contribute</a>
+    <a href="https://github.com/angular/components/blob/master/CONTRIBUTING.md" class="button button-blue" mat-button>
+      Contribute
+    </a>
   </div>
 
   <div class="card-section">
@@ -48,7 +54,9 @@
       to build your Angular app.
 
     </div>
-    <a href="https://github.com/angular/angularfire2/blob/master/CONTRIBUTING.md" class="button button-blue" md-button> Contribute</a>
+    <a href="https://github.com/angular/angularfire/blob/master/CONTRIBUTING.md" class="button button-blue" mat-button>
+      Contribute
+    </a>
   </div>
 
 </article>