message: fix(docs-infra): use up-to-date URLs in "Contribute" links (and replace deprecated `md-button`)
  This commit replaces some outdated URLs in "Contribute" links (on the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
diff: 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">
  </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">
  </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">
  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>
  