Update sprint planning moderation docs

[rdimaio]

https://rucio.github.io/documentation/developer/sprint_planning/#moderating-a-sprint

We now have automatic messages, so we don't need the points about "Announcing the beginning of the sprint in the Mattermost Developers channel. An example message could be: "Sprint planning thread starts! Please announce your availability (in days), and the issues with their estimates - you choose!""

Also, we might want to add info on providing status updates during rucio meetings