Docs: fixes badge and blockquotes syntax

What does this MR do?

• Fixes blockquotes syntax - ports https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/23638 to EE
• Fixes "premium" badge + blockquote syntax for https://gitlab.com/gitlab-com/gitlab-docs/issues/285

Related issues

Closes https://gitlab.com/gitlab-com/gitlab-docs/issues/285

Author's checklist

• Apply the correct labels and milestone
• Correctly apply the product badges and tiers
• Port the MR to EE (or backport from CE): https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/23638

Review checklist

• Technical writer's review (required)
• Merge this EE-MR first, CE-MR afterwards (https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/23638)