Documentation: filesystems: Add bcachefs toctree

Commit eb386617be4bdf ("bcachefs: Errcode tracepoint, documentation")
adds initial bcachefs documentation (private error codes) but without
any table of contents tree for the filesystem docs, hence Sphinx warns:

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

Add bcachefs toctree to fix above warning.

Fixes: eb386617be4b ("bcachefs: Errcode tracepoint, documentation")
Signed-off-by: Bagas Sanjaya <bagasdotme@gmail.com>
Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>