Use feature flag when adding new mailer

What does this MR do and why?

A sort of corrective action/suggestion based on experience with a new mailer that was recently added. The mailer ended up sending way more mail than anticipated and we urgently rolled back the change. Unfortunately, this left Sidekiq in a bad state since the new mailer methods were removed. Ideally we would have taken the time to make the methods no-op, but left them in place.

Another action that would have prevented the need to rollback is the use of a feature flag. Existing docs already allude to a feature flag being recommended because it says to follow Sidekiq docs, which recommend a feature flag. This change adds a small note to email development docs to call attention to this recommendation.

Related discussion https://gitlab.com/gitlab-org/gitlab/-/issues/426316#note_1581949148

Screenshots or screen recordings

Screenshots are required for UI changes, and strongly recommended for all other merge requests.

Before	After

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.