Add developer documentation

[LilleAila]

Jeg har:

• Sett etter lignende issues

Beskriv forslaget

for example for

docker compose build --no-cache

and similar things

[kluvin]

Hijacking this issue, we need significantly more docs to explain the system and how to understand it. We should write these docs fast and first, then get someone to be our guinea pig. I think the frontend can be learnable in a week, and the backend in a few weeks.

What's nice is that everyone has the required domain knowledge to understand what's going on.

I'd like to do something inspired by diataxis

1. https://docs.divio.com/documentation-system/ (better than the official resource, written by the same dude)
2. https://diataxis.fr/