Replace Gitter button with Discord invite, fixes #19626 (#19636)

index.html

@@ -9,7 +9,7 @@ css:
         <h2>Official Go implementation of the Ethereum protocol</h2>
         <br/>
         <a href="https://github.com/ethereum/go-ethereum" target="_blank" class="btn btn-success" style="margin: 4px;"><i class="fa fa-github" aria-hidden="true"></i> View on GitHub</a>
-        <a href="https://gitter.im/ethereum/go-ethereum" target="_blank" class="btn btn-success" style="margin: 4px;"><i class="fa fa-user-circle" aria-hidden="true"></i> Chat on Gitter</a>
+        <a href="https://discord.gg/nthXNEv" target="_blank" class="btn btn-success" style="margin: 4px;"><i class="fa fa-user-circle" aria-hidden="true"></i> Chat on Discord</a>
       </div>
     </div>