Onboarding - Add 15h to budget for onboarding doc improvements

Also separate the onboarding budget's new total of 195h in 3 phases, to ensure the added budget isn't consumed before the new member is accepted in the core team.

See also onboarding-course!81 (merged) and https://tasks.opencraft.com/browse/SE-3761 for the corresponding changes in the onboarding course and the onboarding epic description.

@symbolist @pauloviadanna Would you like to review?