Add custom domain & update repo name

[rzmk]

Currently the domain for the intern guide is set to https://dathere.github.io/de-intern-guide.

We may want to make the following changes:

• Rename the repository to a custom domain name
• Use a custom domain name for the deployed site

I'm not sure what would be a fitting repo/domain name, though some ideas are:

• intern.dathere.com
• interns.dathere.com
• onboard.dathere.com
• guide.dathere.com

Open to discussion/suggestions.