docs: Support Cell social chat.

This MR updates current practice for support cell sprints, as well as adds a procedure for social chats.

https://tasks.opencraft.com/browse/STAR-2866

Author Notes

I've added @gabrieldamours as Sprint Manager backup, since in practice he is usually the person who handles the sprint if I'm away. I'm assuming this should be fine, though this change also introduces a responsibility on support cell sprint managers to handle the social chat by default.

This change also removes the sprint retrospective from the procedure. Since we haven't been doing that, it shouldn't be in the handbook. We typically do some light retrospective work in each synchronous sprint meeting, so this may be sufficient for now, but if needed I could create a follow-up ticket for making a new retrospective procedure. My suspicion is that such a procedure would be better created after moving the team to async so we can more closely mimic how the other team does it (synchronously) without adding an additional meeting.