Draft: Add sentence around weekdays only schedule

To merged when https://gitlab.com/gitlab-com/gl-infra/reliability/-/issues/17058+ is closed.

Why is this change being made?

Add sentence about opt-in only weekday schedule for Incident Managers who cannot be required to work weekends.

Instructions to schedule only weekdays layer: https://docs.google.com/presentation/d/1HCQvMv16XhfztIXHDfJ6l1Mmc8amx0JBQ70q1WwhJpc/edit?usp=sharing

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