Fix most instances of bare URLs in markdown

What does this MR do?

Ports https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/24519 to EE.

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-ce/merge_requests/24519.

Review checklist

• Technical writer's review (required)
• Merge the EE-MR first, CE-MR (https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/24519) afterwards