Break up the 'someos' docs classification based on a more user-focused scheme, including creation of a separate 'Concurrent Execution' section

Doc/library/concurrency.rst

@@ -0,0 +1,31 @@
+.. _concurrency:
+
+********************
+Concurrent Execution
+********************
+
+The modules described in this chapter provide support for concurrent
+execution of code. The appropriate choice of tool will depend on the
+task to be executed (CPU bound vs IO bound) and preferred style of
+development (event driven cooperative multitasking vs preemptive
+multitasking) Here's an overview:
+
+
+.. toctree::
+
+   threading.rst
+   multiprocessing.rst
+   concurrent.futures.rst
+   subprocess.rst
+   sched.rst
+   queue.rst
+   select.rst
+
+
+The following are support modules for some of the above services:
+
+.. toctree::
+
+   dummy_threading.rst
+   _thread.rst
+   _dummy_thread.rst