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 <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Jonathan Corbet <[email protected]>

Author: dwlsalmeida

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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