Documentation/workqueue.txt: convert to ReST markup

... and move to Documentation/core-api folder.

Signed-off-by: Silvio Fricke <silvio.fricke@gmail.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>
diff --git a/Documentation/core-api/index.rst b/Documentation/core-api/index.rst
index ed3eb64..f7ef7fd 100644
--- a/Documentation/core-api/index.rst
+++ b/Documentation/core-api/index.rst
@@ -7,6 +7,8 @@
 .. toctree::
    :maxdepth: 1
 
+   workqueue
+
 .. only::  subproject
 
    Indices