Migrate grml.github.io to grml.org/contribute

[jkirk]

We have a "How to contribute to Grml" site on https://grml.github.io/.

We decided to migrate the content to https://grml.org/contribute (under Development > Contribute), see: https://github.com/grml/grml.org/blob/5248a7ced62ad62de1ecf27642d2f6136cb3bd95/layouts/partials/grml_menu.html#L36

Once we migrated the content, we should decide what content should be on https://grml.github.io/. Redirect to https://grml.org/contribute or put something else on it?