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:

grml.org/layouts/partials/grml_menu.html

Line 36 in 5248a7c

<a class="plain" href="https://grml.github.io/">Contribute</a><br/>

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