Documentation: add a new file documenting multigrain timestamps

Documentation: add a new file documenting multigrain timestamps

Add a high-level document that describes how multigrain timestamps work,
rationale for them, and some info about implementation and tradeoffs.

Reviewed-by: Josef Bacik <josef@toxicpanda.com>
Reviewed-by: Darrick J. Wong <djwong@kernel.org>
Reviewed-by: Randy Dunlap <rdunlap@infradead.org>
Reviewed-by: Jan Kara <jack@suse.cz>
Tested-by: Randy Dunlap <rdunlap@infradead.org> # documentation bits
Signed-off-by: Jeff Layton <jlayton@kernel.org>
Link: https://lore.kernel.org/r/20241002-mgtime-v10-8-d1c4717f5284@kernel.org
Signed-off-by: Christian Brauner <brauner@kernel.org>