GitLab

Bumps [html-proofer](https://github.com/gjtorikian/html-proofer) from 3.18.5 to 3.18.6.
- [Release notes](https://github.com/gjtorikian/html-proofer/releases)
- [Commits](https://github.com/gjtorikian/html-proofer/compare/v3.18.5...v3.18.6)

* Technology service button has been removed from services button
collection so that there are only 4 instead of 5 buttons.

* Add the contact email link for GitLab inquiries to the GitLab Services
overview page.

* Rephrasing some sentences according to review.

* Contact mail link can be set in cases where it is divergating from
variable contact_mail.

* Additional variable serves as contact email address that includes the
email subject already in the link.

* Desktop: rows of three, aligned to the right.
* Tablet: rows of three, centered.
* Phone: rows of two, centered.

Inporporate review suggestions:
* Exchange plain links by buttons pointing to further documentation.
* Exchange links by links of GitLab help pages.
* Add more links to GitLab features.

* Link of GitLab button in button collection in top header should point
to GitLab service page directly.

* Undraw images can be customized with Helmholtz colors.

* Link GitLab docs in each section on the Helmholtz GitLab Service page
about the respective topic.

* Name the respective programming languages of the package reigstries
listed.

* Explain why GitLab has been chosen.
* Explain most useful GitLab features.
* Introduce Helpdesk to get in touch.

* CSS file to layout service page can be reused.
* 2 undraw images need to be replaced.

* CSS file now has a name with less context information and can be
reused in other contexts.

* Exchange images for section GitLab CI and GitLab Package Registry.

* Some images are unused and thus can be removed.

* Existing css file can be reused in this context.
* Rename css file to remove the context information.

* Add unsplash images that show up as title images scaled for different
devices.

* Add a couple of undraw images that decorate the service page nicely.

    * Additional button that points to descriptions of the Helmholtz GitLab
    service.
    * Add some css to layout button next to the other services buttons on
    different devices.
    * Add some css to layout service page.
    * Add some dummy text to see how it looks like if text is included.