Document feature flag creation process

[stvnrlly]

Following #1949, we should document our process over in https://github.com/GSA/notifications-api/blob/main/docs/all.md while it's fresh.

It may fit best in/near the "Deploying" section.

[alexjanousekGSA]

@stvnrlly We documentation here https://github.com/GSA/notifications-admin/blob/main/docs/feature-flagging.md What was your vision for updating the "all" readme? Is the goal to create a more indepth version of this feature-flagging.md or do you want to move all of the content there and then enhance it with even more details? Do we want two different documents?