message: docs(button): update demo design
diff: --ion-safe-area-top: 20px;
  --ion-safe-area-bottom: 22px;
  }
  +
  + header {
  + font-size: 11px;
  + font-weight: 600;
  + letter-spacing: 0.05em;
  + margin-bottom: 1.25em;
  + text-transform: uppercase;
  + }
  +
  + section {
  + margin-top: 1em;
  + margin-bottom: 2.5em;
  + }
  +
  + section:not(.full-width),
  + .full-width > header {
  + padding: 0 10px;
  + }
  </style>
  </head>
  <body>
  <ion-toolbar>
  <ion-title>Button</ion-title>
  </ion-toolbar>
  - </ion-header>,
  - <ion-content fullscreen padding>
  - <p><ion-button color="light">Light</ion-button></p>
  - <p><ion-button>Default</ion-button></p>
  - <p><ion-button color="secondary">Secondary</ion-button></p>
  - <p><ion-button color="danger">Danger</ion-button></p>
  - <p><ion-button color="dark">Dark</ion-button></p>
  + </ion-header>
  + <ion-content fullscreen>
  +
  + <section>
  + <header>Small Size</header>
  + <ion-button size="small">Default</ion-button>
  + <ion-button size="small" color="secondary">Secondary</ion-button>
  + <ion-button size="small" color="tertiary">Tertiary</ion-button>
  + </section>
  +
  + <section>
  + <header>Default Size</header>
  + <ion-button color="success">Success</ion-button>
  + <ion-button color="warning">Warning</ion-button>
  + <ion-button color="danger">Danger</ion-button>
  + </section>
  +
  + <section>
  + <header>Large Size</header>
  + <ion-button size="large" color="light">Light</ion-button>
  + <ion-button size="large" color="medium">Medium</ion-button>
  + <ion-button size="large" color="dark">Dark</ion-button>
  + </section>
  +
  + <section>
  + <header>Block Width</header>
  + <ion-button expand="block">A block button</ion-button>
  + </section>
  +
  + <section class="full-width">
  + <header>Full Width</header>
  + <ion-button expand="full" color="secondary">A full-width button</ion-button>
  + </section>
  +
  </ion-content>
  </ion-app>
  </body>
  