Added link to tutorial for Thread class

doc/classes/Mutex.xml

@@ -7,6 +7,7 @@
 		A synchronization mutex (mutual exclusion). This is used to synchronize multiple [Thread]s, and is equivalent to a binary [Semaphore]. It guarantees that only one thread can ever acquire the lock at a time. A mutex can be used to protect a critical section; however, be careful to avoid deadlocks.
 	</description>
 	<tutorials>
+		<link>https://docs.godotengine.org/en/latest/tutorials/threads/using_multiple_threads.html</link>
 	</tutorials>
 	<methods>
 		<method name="lock">

doc/classes/Semaphore.xml

@@ -7,6 +7,7 @@
 		A synchronization semaphore which can be used to synchronize multiple [Thread]s. Initialized to zero on creation. Be careful to avoid deadlocks. For a binary version, see [Mutex].
 	</description>
 	<tutorials>
+		<link>https://docs.godotengine.org/en/latest/tutorials/threads/using_multiple_threads.html</link>
 	</tutorials>
 	<methods>
 		<method name="post">

doc/classes/Thread.xml

@@ -7,6 +7,7 @@
 		A unit of execution in a process. Can run methods on [Object]s simultaneously. The use of synchronization via [Mutex] or [Semaphore] is advised if working with shared objects.
 	</description>
 	<tutorials>
+		<link>https://docs.godotengine.org/en/latest/tutorials/threads/using_multiple_threads.html</link>
 	</tutorials>
 	<methods>
 		<method name="get_id" qualifiers="const">