Add sparst.rst to toctree, so it can be part of the docs build.

Cc: Randy Dunlap <rdunlap@infradead.org>
Cc: Jonathan Corbet <corbet@lwn.net>
Suggested-by: Jonathan Corbet <corbet@lwn.net>
Signed-off-by: Min-Hua Chen <minhuadotchen@gmail.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>
Link: https://lore.kernel.org/r/20230902052512.12184-4-minhuadotchen@gmail.com