Documentation/assoc_array.txt: convert to ReST markup

... and move to Documentation/core-api folder.
Signed-off-by: NSilvio Fricke <silvio.fricke@gmail.com>
Reviewed-by: NMauro Carvalho Chehab <mchehab@s-opensource.com>
Signed-off-by: NJonathan Corbet <corbet@lwn.net>

Documentation/core-api/index.rst

@@ -7,6 +7,7 @@ Kernel and driver related documentation.
 .. toctree::
    :maxdepth: 1
 
+   assoc_array
    workqueue
 
 .. only::  subproject