Add tutorial: Migrate domains, websites, mails and databases to Hetzner web hosting

[Martin555]

I have read and understood the Contributor's Certificate of Origin available at the end of https://raw.githubusercontent.com/hetzneronline/community-content/master/tutorial-template.md and I hereby certify that I meet the contribution criteria described in it. Signed-off-by: [Martin Badstübner [email protected]]

Please have a look at the header "cta"...I'm not quite sure about it.