Update file sidekiq-survival-guide-for-sres.md

This MR adds a section to A survival guide for SREs to working with Sidekiq at GitLab with a reference to https://gitlab.com/gitlab-com/runbooks/-/blob/master/docs/sidekiq/deferring-jobs.md#2-using-feature-flags-via-chatops

Rendered markdown: https://gitlab.com/gitlab-com/runbooks/-/blob/9a5af81fc1fed1ad53a2ace93959150f12e777d7/docs/sidekiq/sidekiq-survival-guide-for-sres.md#deferring-sidekiq-jobs