The purpose of an R&D Doc is to explore architecture, parts, views and components for an application.
This is not waterfall, it is a living and breathing doc that will evolve as the project evolves.
It is focused more on the lower fidelity high level design architecture, views and components then the final UI.
When possible It sketches out parts and components in language agnostic way specifying inputs and output “interfaces” and leaves technical implantation dependent on framework and language of choice to a second draft.
It’s also valuable to evaluate and get feedback on implementation from others.
It might also help to define a taxonomy of your system as list or diagram to define terminology used across the doc.