docs: document why we chose plantuml + drawio #183

PlantUML is being introduced in #158, however there is no explanation why PlantUML was chosen. We also support Draw.IO, again without any explanation why Draw.IO was chosen.

We need to document the decision, even if it's too late.

In case score process does not offer any advice on how to do that, have a look at https://github.com/joelparkerhenderson/architecture-decision-record

Note: it does not need to have an unreasonable level of detail or fake arguments. If it's a simple reason, document the simple reason and that's it.