move threads section to performance to match master branch

tutorials/performance/index.rst

@@ -86,3 +86,13 @@ GPU
    :name: toc-learn-features-3d-optimization
 
    optimizing_3d_performance
+
+Multi-threading
+--
+
+.. toctree::
+   :maxdepth: 1
+   :name: toc-learn-features-threads
+
+   threads/thread_safe_apis
+   threads/using_multiple_threads

tutorials/threads/index.rst

@@ -1,9 +0,0 @@
-Multi-threading
-===============
-
-.. toctree::
-   :maxdepth: 1
-   :name: toc-learn-features-threads
-
-   using_multiple_threads
-   thread_safe_apis