docs: add guidelines for submitting new filesystems

This document is motivated by the ongoing maintenance burden that
abandoned and untestable filesystems impose on VFS developers, blocking
infrastructure changes such as folio conversions and iomap migration.

This week alone, two new filesystems were proposed on linux-fsdevel
(VMUFAT and FTRFS), highlighting the need for documented guidelines
that new filesystem authors can refer to before submission.

Multiple recent discussions on linux-fsdevel have touched on the
criteria for merging new filesystems and for deprecating old ones,
covering topics such as modern VFS interface adoption, testability,
userspace utilities, maintainer commitment, and user base viability.

Add Documentation/filesystems/adding-new-filesystems.rst describing
the technical requirements and community expectations for merging a
new filesystem into the kernel.  The guidelines cover:
- Alternatives to consider before proposing a new in-kernel filesystem
- Technical requirements: modern VFS interfaces (iomap, folios,
  fs_context mount API), testability, and userspace utilities
- Community expectations: identified maintainers, demonstrated
  commitment, sustained backing, and a clear user base
- Ongoing obligations after merging, including the risk of deprecation
  for unmaintained filesystems

Link: https://lore.kernel.org/linux-fsdevel/20260411151155.321214-1-adrianmcmenamin@gmail.com/
Link: https://lore.kernel.org/linux-fsdevel/20260413142357.515792-1-aurelien@hackers.camp/
Link: https://lore.kernel.org/linux-fsdevel/yndtg2jbj55fzd2kkhsmel4pp5ll5xfvfiaqh24tdct3jiqosd@jzbfzf3rrxrd/
Link: https://lore.kernel.org/linux-fsdevel/20260124091742.GA43313@macsyma.local/
Link: https://lore.kernel.org/lkml/20260111140345.3866-1-linkinjeon@kernel.org/
Cc: Christian Brauner <brauner@kernel.org>
Cc: Alexander Viro <viro@zeniv.linux.org.uk>
Cc: Jan Kara <jack@suse.cz>
Cc: Theodore Tso <tytso@mit.edu>
Cc: Christoph Hellwig <hch@lst.de>
Cc: Darrick J. Wong <djwong@kernel.org>
Cc: Matthew Wilcox <willy@infradead.org>
Assisted-by: Cursor:claude-4-opus
Signed-off-by: Amir Goldstein <amir73il@gmail.com>
Link: https://patch.msgid.link/20260422125212.1743006-1-amir73il@gmail.com
Reviewed-by: Jan Kara <jack@suse.cz>
Signed-off-by: Christian Brauner (Amutable) <brauner@kernel.org>

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