Create documentation for (dev) environment setup

[jbtfk]

Add documentation to get a basic development environment up and running

Remaining tasks

• [ ] Required PHP extensions (Possibly for all sinks)
• [ ] Document example config in /etc, utilized in next steps:
• [ ] Setting up Nginx and use the provided nginx config.
• [ ] Setting up cron run (remember permission: 0600)
• [ ] Setting up supervisord and queue jobs through it
• [ ] Pub/sub mechanisms with soketi (or similar)
• [ ] Add notice/remark regarding working with local package development (symlinking composer dependencies)