Enhance DEVELOPING guide with full-stack local setup instructions

[noraeb]

As a contributor to CoMPAS,
I want to have clear and structured documentation on how to run all services locally and understand how the different components relate to each other,
So that I can efficiently set up a local development environment and contribute to individual services without confusion or unnecessary setup time.

Acceptance criteria

• A high-level overview of the architecture is included or linked, showing how the services interact
• Update CoMPAS architecture (if not too much work) where needed
• A step-by-step guide is provided to run the entire CoMPAS stack locally using Docker Compose (or other tooling, if needed)
• Instructions are provided for running an individual service locally
• Common environment variables, ports, and dependencies are documented
• Links to relevant READMEs and docs are included + those docs are up to date

The documentation is added to or linked from the existing DEVELOPING.html page