Feature hugo migration

This MR converts the existing website into a hugo website.

• Converting existing pages into a hugo website
• Existing .html links should work
• Atom feed: Still has validation errors
• html5: Fix all validation errors
• Hide emails from spam bots
• rel="noreferrer" attribute missing on all links
• rel="noopener" attribute missing on all links

Have a look at README.md how to test it with hugo's internal webserver.

This is my very first MR - feedback is very welcome.

If this is not the right way to review and discuss, please let me know.

Closes #1 (closed)