docs: index.rst: Add watch_queue

Fix the following sphinx warning:

Documentation/watch_queue.rst:
WARNING: document isn't included in any toctree

By adding watch_queue.rst to the index.
Signed-off-by: Daniel W. S. Almeida <dwlsalmeida@gmail.com>
Link: https://lore.kernel.org/r/20200718165107.625847-13-dwlsalmeida@gmail.comSigned-off-by: Jonathan Corbet <corbet@lwn.net>

Documentation/index.rst

@@ -193,6 +193,7 @@ to ReStructured Text format, or are simply too old.
    :maxdepth: 2
 
    staging/index
+   watch_queue
 
 
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