docs: sharing guide: clarify end-game of "Example 2" - push to dev-repo, rather than pulling into it (consistent with how we work the multiple developer scenario later) - explicitly demonstrate that public changesets are immutable

docs: minor tweaks to sharing guide (suggested by timeless)

docs: add code review scenario to sharing guide The idea is to demonstrate a simpler multiple-developer situation that does not involve getting into trouble. The final scenario illustrates Alice and Bob getting into trouble with bumped and divergent changesets by amending each other's history. The required tests and text are all written, but will need to be heavily revised because of the inserted scenario.

docs: add a bit more hierarchy to the sharing guide