Documentation: ext4: Reduce toctree depth

Reduce toctree depth from 6 to 2 to only show individual docs titles
on top-level toctree (index.rst) and to not spoil the entire hierarchy.

Signed-off-by: Bagas Sanjaya <[e-mail]>
Acked-by: Theodore Ts'o <[e-mail]>
Acked-by: Darrick J. Wong <[e-mail]>
Signed-off-by: Jonathan Corbet <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]

Documentation/filesystems/ext4/index.rst

···
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 .. toctree::
-   :maxdepth: 6
+   :maxdepth: 2
    :numbered:
 
    about