add blurb about check-in call

Why is this change being made?

I added in a blurb about check-in call to the handbook since it's not listed anywhere. If we are going to really start pushing for this call to work, we should probably have it in the handbook with an explanation of the purpose.

I liked Bev's idea about adding a message into the new team member update channel and we can update the handbook with that info too.

cc @gl-people-exp

Author Checklist

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