Documentation: New Hook Pattern

[jmbrunskill]

Add some documentation about how to use and migrate to the new hook pattern.

A link to a reference example for how we should do things now in code base would be useful

Could be something like these docs for dsl changes

https://github.com/msupply-foundation/open-msupply/wiki/Code-Review-Comments---Rust#prefer-using-diesel-table-modules-over-dsl

Could also add a link to https://github.com/msupply-foundation/open-msupply/wiki/Refactoring#long-term-refactoring

Maybe also for the diesel one? So have a list of things we're trying to work though...