Migrate content to jekyll (part 1)

Following the example of
https://signalvnoise.com/posts/3418-teaching-the-support-team-how-to-fis
h we will try to maintain this site via
https://github.com/jekyll/jekyll.

This is the first big chunk, all pages are working.
Config and redirects are still missing. Also some jekyll default files
are still lying around.

This includes some minor content and css changes.
jekyll cleanup and config

Also some redirecting for old content.
Add OG-tags and twitter-card
Rename and move images to the right place

Cleaning up the unreadable p2h-image-names.
Redirect / to /beratung/
Cleanup
Add dev note