PD: document how dynamic step edit form fields work

[IanLondon]

PD's "dynamic" step edit forms are only presently used in the Thermocycler profile step. Since users can arbitrarily add and remove these fields, the typical "hard-wire the name into each UI component" approach we use for non-dynamic fields doesn't quite work here. For example, the way we do pristinity requires FocusHandlers to be passed down all the way into the component, and different utilities are used eg showProfileFieldErrors instead of showFieldErrors.

Since these fields function differently, they deserve documentation that outlines what goes into these fields and how they differ from normal non-dynamic form fields in PD.