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.com


Signed-off-by: Jonathan Corbet <corbet@lwn.net>