Improve homepage, document mattermost and point on the official website.

BTCPayServer/Views/Home/Home.cshtml

@@ -10,7 +10,7 @@
             <h1>Welcome to BTCPay Server</h1>
             <hr />
             <p>BTCPay Server is a free and open source server for merchants wanting to accept Bitcoin for their business.</p>
-            <a style="background-color: #fff;color: #222;display:inline-block;text-align: center;white-space: nowrap;vertical-align: middle;user-select: none;line-height: 1.25;font-size: 1rem;text-decoration:none;font-weight: 700;    text-transform: uppercase;border: none;border-radius: 300px;padding: 15px 30px;" href="https://docs.btcpayserver.org">Getting started</a>
+            <a style="background-color: #fff;color: #222;display:inline-block;text-align: center;white-space: nowrap;vertical-align: middle;user-select: none;line-height: 1.25;font-size: 1rem;text-decoration:none;font-weight: 700;    text-transform: uppercase;border: none;border-radius: 300px;padding: 15px 30px;" href="https://btcpayserver.org" target="_blank">Official website</a>
         </div>
     </div>
 </header>
@@ -128,13 +128,19 @@
             </div>
         </div>
         <div class="row">
-            <div class="col-lg-4 ml-auto text-center">
-                <a href="http://slack.forkbitpay.ninja/">
+            <div class="col-lg-3 ml-auto text-center">
+                <a href="https://chat.btcpayserver.org/">
+                    <img src="~/img/mattermost.png" height="100" />
+                </a>
+                <p><a href="https://chat.btcpayserver.org/">On Mattermost</a></p>
+            </div>
+            <div class="col-lg-3 ml-auto text-center">
+                <a href="https://slack.btcpayserver.org/">
                     <img src="~/img/slack.png" height="100" />
                 </a>
                 <p><a href="http://slack.forkbitpay.ninja/">On Slack</a></p>
             </div>
-            <div class="col-lg-4 mr-auto text-center">
+            <div class="col-lg-3 mr-auto text-center">
                 <a href="https://twitter.com/BtcpayServer">
                     <img src="~/img/twitter.png" height="100" />
                 </a>
@@ -142,7 +148,7 @@
                     <a href="https://twitter.com/BtcpayServer">On Twitter</a>
                 </p>
             </div>
-            <div class="col-lg-4 mr-auto text-center">
+            <div class="col-lg-3 mr-auto text-center">
                 <a href="https://github.com/btcpayserver/btcpayserver">
                     <img src="~/img/github.png" height="100" />
                 </a>