Add gitlab-ui to the docs codebase

Since #407 (closed) we support modern JS which also means we can start using our component library https://gitlab.com/gitlab-org/gitlab-ui

What is gitlab-ui?

gitlab-ui is a component library. It contains re-usable components that we use in the GitLab product. If a component is changed in gitlab-ui, it means that all instances of the component will be updated in the GitLab product as well. The gitlab-ui component library helps us by ensuring consistency, reduce development time and helps with the maintainability of the codebase.

Where can I see all of the available components?

All of the components in the gitlab-ui library can be viewed at https://gitlab-org.gitlab.io/gitlab-ui

Benefits

• Re-use the components in gitlab-ui instead of building docs-specific components.

• Aligns the docs UI with our product UI (little to no visual regression between the two UI's).

• Aligns the docs codebase with the product.

• We'll use well-tested components which are less prone to bugs.

• Makes the docs codebase more maintainable.

• etc.

Limitations

gitlab-ui is a Vue based component library, this means that the components can only be used within a Vue context. The docs project is primarily eRuby which means that we cannot import/use components from gitlab-ui without converting the page, or pieces of a page to Vue.