Backport documentation from
https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/15580

Ports changes from
https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/12343

Marcel Amirault authored 5 years ago and Achilleas Pipinellis committed 5 years ago

Previously, we used brackets to denote the tier badges,
but this made Kramdown, the docs site Markdown renderer,
show many warnings when building the site. This is now
fixed by using parentheses instead of square brackets.

This was caused by [PREMIUM] looking like a link to
Kramdown, which couldn't find a URL there.

See:
- https://gitlab.com/gitlab-com/gitlab-docs/merge_requests/484
- https://gitlab.com/gitlab-org/gitlab-ce/issues/63800

For ssot epic