Reformat new MailRoom content for consistency with other documentation

What does this merge request do and why?

Follow up to: !2827 (merged), mostly for formatting.

• Before: https://gitlab.com/gitlab-org/gitlab-development-kit/-/blob/85d8fd3b2b239c70bffa957f5a0728908d15a38f/doc/howto/service_desk_mail_room.md
• After: https://gitlab.com/gitlab-org/gitlab-development-kit/-/blob/2f4f57e9c2686c006fca5697befd14eb64de018d/doc/howto/service_desk_mail_room.md

CC @danmh

Impacted categories

The following categories relate to this merge request:

• gdk-reliability - e.g. When a GDK action fails to complete.
• gdk-usability - e.g. Improvements or suggestions around how the GDK functions.
• gdk-performance - e.g. When a GDK action is slow or times out.

Merge request checklist

• This change is backward compatible. If not, please include steps to communicate to our users.
• Tests added for new functionality. If not, please raise an issue to follow-up.
• Documentation added/updated, if needed.
• gdk doctor test added, if needed.
• Add the ~highlight label if this MR should be included in the CHANGELOG.md.