Documentation: ext4: blockgroup: Add explicit title heading

Block groups documentation has three, first-level section headings.
These headings' text become toctree entries and the first one "Layout"
becomes docs title in the output, which isn't conveying the docs
contents.

Add explicit title heading and demote the rest.

Signed-off-by: Bagas Sanjaya <bagasdotme@gmail.com>
Acked-by: Theodore Ts'o <tytso@mit.edu>
Acked-by: Darrick J. Wong <djwong@kernel.org>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>
Link: https://lore.kernel.org/r/20250620105643.25141-6-bagasdotme@gmail.com

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