Add first-day-of-week admin area config note

What does this MR do?

Completes the docs for https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/22755, until the admin area is fully documented.

Related issues

Related to: https://gitlab.com/gitlab-org/gitlab-ce/issues/57874.

Author's checklist

• Apply the correct labels and milestone

Review checklist

• Your team's review (required)
• PM's review (recommended, but not a blocker)
• Technical writer's review (required)