Documentation: bcachefs: Add casefolding toctree entry

Sphinx reports htmldocs toctree warning:

Documentation/filesystems/bcachefs/casefolding.rst: WARNING: document isn't included in any toctree

Fix the warning by adding casefolding documentation entry to bcachefs
toctree.

Fixes: bc5cc09246c5 ("bcachefs: bcachefs_metadata_version_casefolding")
Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Closes: https://lore.kernel.org/linux-next/20250221161728.32739f85@canb.auug.org.au/


Signed-off-by: Bagas Sanjaya <bagasdotme@gmail.com>
Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>