Clarify why demos and demo prep are important

I hear this comment today:
> I don’t think [the demo] requires much prep as our client has been following all the way through, demos are more important when the client doesn’t attend the standups

It was for a small project with a fantastic dwyl team and a lovely client, so the impact is not big for _this_ project, but it reminded me that I _clearly_ have done a terrible job communicating the importance of _why_ we do demo prep and demos.

Start by reviewing the existing process-handbook guidance on demos and demo preparation, then compare it with the concern quoted in this issue. Done means the handbook clearly explains why preparation and demos matter, including when client attendance or standups do not provide the same value.

Written by the indexing model from the issue text.