Update onboarding guide to be consistent with GitFlow processes

[stiber]

See https://docs.google.com/document/d/12MC96qsg2-201DkNf9vs2hqpAJwcG2vkb-fFXGHh4Nk/edit?tab=t.0#heading=h.h2h56vktodpq

[stardriftfx]

Version 1: See my suggestions and release notes here: https://docs.google.com/document/d/12MC96qsg2-201DkNf9vs2hqpAJwcG2vkb-fFXGHh4Nk/edit?tab=t.0

@stiber What do you think?

[stardriftfx]

Note: Closed #892 because it's a duplicate issue of this one.

@stiber The onboarding guide should be all set for now.