Resolve "New service page regarding Helmholtz GitLab"

Dear Tobias @frust45

in order to get some early feedback on the way I hooked in a new service page I would like to get some first feedback on:

• Layout of services buttons in page header on different devices,
• Teaser of GitLab service on services page,
• New service page structure and its layout,
• Unsplash images,
• Undraw images,
• Implementation and cleanliness.

I will start working on the actual content when I got you okay that I can continue in this direction. Many thanks in advance.

Closes #208 (closed)