Documentation: bcachefs: Split index toctree

bcachefs subsystem currently has 4 docs: two are development notes and
the rest are actual filesystem docs. These two groups are clearly
distinct and can be organized.

Split the toctree into two, one for each docs group. While at it, also
reduce :maxdepth: so that only title headings are listed in the
toctrees.

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