Modify handbook/change request template when timing is important and devs are requested for assistance

As part of https://gitlab.com/gitlab-com/gl-infra/infrastructure/issues/6027, sometimes we do work where it may be necessary to have other engineers readily available to assist in times of need.

Utilize this issue to track adding language to both our handbook and our change issue request template to ensure that we've gathered the right people to help when certain types of changes require that we need a certain skillset readily available.