Fix docfx warning re: index.html links (akkadotnet#3707)

* Update serialization.md

Fixes warning: [19-02-06 08:40:50.533]Warning:[BuildCommand.BuildCore.Build Document.CompilePhaseHandlerWithIncremental.ConceptualDocumentProcessor.Build.BuildConceptualDocument](articles/networking/serialization.md#L197)Inline code snippet is only allowed inside tables.

* Update index.md

docs/index.md

@@ -28,7 +28,7 @@ h2:before{
       <h1 class="title">Try <strong>Akka.NET</strong> now!</h1>
       <h1 class="title"><small class="subtitle">Follow our tutorial and build your first Akka.NET application today.</small></h1>
       <div class="options">
-        <a class="btn btn-lg btn-primary" href="articles/intro/tutorial-1.html">Get Started Now</a> <a class="btn btn-lg btn-primary" href="articles/intro/what-is-akka.html">Read the documentation</a>
+        <a class="btn btn-lg btn-primary" href="articles/intro/tutorial-1.md">Get Started Now</a> <a class="btn btn-lg btn-primary" href="articles/intro/what-is-akka.md">Read the documentation</a>
       </div>
     </div>
 </div>