[$] Converting filesystems to iomap

A discussion that largely centered around the documentation ofiomap, which provides a block-mapping interface for modern filesystems,was led by Luis Chamberlain at the2023 Linux Storage, Filesystem,Memory-Management and BPF Summit. There is an ongoing process ofconverting filesystems to use iomap, in order to leave buffer heads behind and to better support folios, sothe intent was to get feedback on the documentation from developers who areworking on those conversions. One of the concrete outcomes of the sessionwas a plan to move that documentation from its current location on theKernelNewbies wiki intothe kernel documentation.