Update guide for codespaces, remove docker-in-docker

[mandrasch]

• After ddev/ddev#5267 is merged, guide on homepage needs an update as well
• docker-in-docker is not needed anymore if the universal image is used

Guide: https://ddev.com/get-started/

Don't have the time resources in the next days to dive into astro (missed the contributor training unfortunately) and provide a PR, but I could provide it in the next weeks.

Just wanted to create the issue so that this does not get lost. Cheers!