Documentation: fuse: Consolidate FUSE docs into its own subdirectory

All four FUSE docs are currently in upper-level
Documentation/filesystems/ directory, but these are distinct as a group
of its own. Move them into Documentation/filesystems/fuse/ subdirectory.

Signed-off-by: Bagas Sanjaya <bagasdotme@gmail.com>
Acked-by: Jeff Layton <jlayton@kernel.org>
Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>