Expand Calendly advice to multi-colleague scheduling
This is an adaptation of the "Auto block PagerDuty shifts in Calendly" section, but with a focus on multiple GitLab colleagues' calendars, for cases where several from our side need to attend a customer call.
cc @jgianoutsos @mlockhart & @anton due to !79744 (merged) & !77144 (merged)
Edited by Katrin Leinweber