docs(website): Added a new "troubleshooting" section with "common mistakes" subsection

Author: andgeno

docs/_data/docs.yml

@@ -35,4 +35,8 @@
   - custom-transports
   - network-profiler-window
   - custom-serialization
-  - boxing-systems
+  - boxing-systems
+
+- title: Troubleshooting
+  docs:
+  - common-mistakes

docs/_docs/troubleshooting/common-mistakes.md

@@ -0,0 +1,30 @@
+---
+title: Common Mistakes
+permalink: /wiki/common-mistakes/
+---
+
+This is a collection of common mistakes:
+
+- [`NullReferenceException` when trying to start a server/host/client](#err-001)
+
+---
+
+### <a name="err-001"></a>`NullReferenceException` when trying to start a server/host/client
+
+#### Problem
+When trying to start a server, host, or client by executing one of these lines of code
+
+```csharp
+NetworkingManager.Singleton.StartServer()
+NetworkingManager.Singleton.StartHost()
+NetworkingManager.Singleton.StartClient()
+```
+
+the following exception is thrown:
+
+```csharp
+NullReferenceException: Object reference not set to an instance of an object
+```
+
+#### Solution
+You most likely forgot to add the `NetworkingManager` component to a game object in your scene.