Add section on managing time spent on Slack

Why is this change being made?

Inspired by the All-Remote office hours and my conversation with @jessicareeder today, I've added some practices that folks can use to protect their own time.

Does this MR meet the acceptance criteria?

Assign to DRI

• Did you assign this change to the correct DRI of the page or information you are changing?

Conformity

• Added description to this MR explaining the reasons for the proposed change, per say-why-not-just-what