Documentation: filesystems: Add bcachefs toctree

Commit eb386617 ("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: eb386617 ("bcachefs: Errcode tracepoint, documentation")
Signed-off-by: Bagas Sanjaya <bagasdotme@gmail.com>
Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>

Documentation/filesystems/bcachefs/index.rst

+.. SPDX-License-Identifier: GPL-2.0
+
+======================
+bcachefs Documentation
+======================
+
+.. toctree::
+   :maxdepth: 2
+   :numbered:
+
+   errorcodes

Documentation/filesystems/index.rst

@@ -69,6 +69,7 @@ Documentation for filesystem implementations.
    afs
    autofs
    autofs-mount-control
+   bcachefs/index
    befs
    bfs
    btrfs