Move reviewer roulette table above and improve text

What does this MR do?

Moves the roulette table towards the top of the section and add concise instructional text before it.

Related issue: https://gitlab.com/gitlab-org/quality/team-tasks/-/issues/894

Screenshots (strongly suggested)

Conformity

• I have included a changelog entry, or it's not needed. (Does this MR need a changelog?)
• I have added/updated documentation, or it's not needed. (Is documentation required?)
• I have properly separated EE content from FOSS, or this MR is FOSS only. (Where should EE code go?)
• I have added information for database reviewers in the MR description, or it's not needed. (Does this MR have database related changes?)
• I have self-reviewed this MR per code review guidelines.
• This MR does not harm performance, or I have asked a reviewer to help assess the performance impact. (Merge request performance guidelines)
• I have followed the style guides.