[STAR-2989] Consolidating and updating admin recruitment doc
This MR:
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Moves the contents of billing_onboarding.md and contract_new_hire.md (previously found outside the handbook, at documentation/public/tutorials/howtos) to new sections in recruitment.md, to consolidate all admin-related recruitment instructions and make them available in the handbook.
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Updates recruitment.md to reflect how billing is managed for trial projects.
Edited by Gabriel D'Amours