[STAR-2989] Consolidating and updating admin recruitment doc

This MR:

• 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.

• Updates recruitment.md to reflect how billing is managed for trial projects.