Documentation: the design of iomap and how to port

Capture the design of iomap and how to port filesystems to use it.
Apologies for all the rst formatting, but it's necessary to distinguish
code from regular text.

A lot of this has been collected from various email conversations, code
comments, commit messages, my own understanding of iomap, and
Ritesh/Luis' previous efforts to create a document.  Please note a large
part of this has been taken from Dave's reply to last iomap doc
patchset. Thanks to Ritesh, Luis, Dave, Darrick, Matthew, Christoph and
other iomap developers who have taken time to explain the iomap design
in various emails, commits, comments etc.

Cc: Dave Chinner <[email protected]>
Cc: Matthew Wilcox <[email protected]>
Cc: Christoph Hellwig <[email protected]>
Cc: Christian Brauner <[email protected]>
Cc: Ojaswin Mujoo <[email protected]>
Cc: Jan Kara <[email protected]>
Cc: Luis Chamberlain <[email protected]>
Inspired-by: Ritesh Harjani (IBM) <[email protected]>
Signed-off-by: Darrick J. Wong <[email protected]>
Link: https://lore.kernel.org/r/20240614214347.GK6125@frogsfrogsfrogs
Signed-off-by: Christian Brauner <[email protected]>

Author: Darrick J. Wong

Documentation/filesystems/index.rst

@@ -34,6 +34,7 @@ algorithms work.
    seq_file
    sharedsubtree
    idmappings
+   iomap/index
 
    automount-support