docs: update sharing guide based (mostly) on marmoute's review - don't claim certain scenarios are more/less common, just simple/advanced - mention code review as a multiple-developer scenario (not described in detail yet!) - suggest "hg config --edit --local" instead of "cat >> .hg/hgrc" - use -q less often (and show resulting output) - edit some section headers to be consistent with user guide (example numbers; "amend" instead of "amending") (These are just the small changes; big changes are yet to come.)

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