Update editors chapter with dashboard

[mpadge]

Including describing how dashboard status is taken from slack, so editors can set their slack status to away to update that on dashboard.

[noamross]

We need to coordinate on how this should work before documenting further implementation or documentation. We have implemented separate editor status systems and we shouldn't have two.

[mpadge]

I'm working on resolving that within the dashboard. Airtable will be the primary status, and slack will feed through to that in one direction only. Docs won't be updated until that all works.