doc: revamp contribution guide ahead of contributing live-stream

A fairly significant revamp of the contribution guide ahead of a
community-focussed contribution live stream. I will address bugs,
suggested improvements etc in subsequent CLs after the live stream
(which might well highlight further changes).

Signed-off-by: Paul Jolly <paul@myitcv.io>
Change-Id: I75dae25385a79515a89348401e4ac373f3c5943c
Reviewed-on: https://review.gerrithub.io/c/cue-lang/cue/+/527030

Author: myitcv