Initial how-to guide of implementing deposit price adjuster

[bjarnef]

Just some initial documentation of a deposit price adjustment.

[mattbrailsford]

Some things to be fixed, but also some misunderstanding of how price adjustments work.

Also, might be worth reviewing the updates to the price adjustments key concept docs and see if this is now covered by those docs, or whether we think this how to guide is still useful.

[bjarnef]

@mattbrailsford I have updated the docs. I think it would be a great supplement to the basic key concept of a concrete use-case, where you want to add a deposit, e.g. when with rental of producs or maybe pre-order an expensive product?