Remove consecutive blank lines from markdown files

What does this MR do?

As part of https://gitlab.com/gitlab-com/gitlab-docs/issues/287, remove unnecessary consecutive blank lines from the Markdown files in the doc suite.

Related issues

Relates to: https://gitlab.com/gitlab-com/gitlab-docs/issues/287

Author's checklist

• Apply the correct labels and milestone
• Port the MR to EE (or backport from CE): https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/9568

Review checklist

• Technical writer's review (required)
• Merge the EE-MR first (https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/9568), CE-MR afterwards