Added the Payroll specification

Internal: added docs/specs/PAYROLL.md, the version 1 specification for a Payroll area: employees with hourly or salaried pay on pay schedules; hours from Time Tracking, published Staff Scheduling shifts or entered by hand, with overtime from the team's own rule; earnings, deductions and employer contributions the team defines; pay runs that are approved, frozen and marked as paid; payslips with year-to-date totals that employees read with their own Magpie account; postings to Accounting; and a payroll summary with a CSV. Statutory tax calculation, paying employees, plans and limits, and four other decisions are open for the owner. Nothing is built yet.