Delete overview.serlo.org and archive repos for it

[AndreasHuber]

Repos:

• https://github.com/serlo/vm-configuration
• https://github.com/serlo/overview.serlo.org

RICE Score

• Reach and impact: 2 -- Nobody will really notice this in their day-to-day work but there will be less repos in the Serlo GitHub organization, and there will be fewer modules in Terraform.
• Confidence: 10 -- We have full control over our GitHub.
• Effort: 1 -- Archiving GitHub repos is just a couple of clicks.