docs: add tax rules, scoped voucher lookup and gap drill-down to the tutorial

Two new chapters bring the guide to 108 steps over 13 chapters:

- قواعد الضرائب: entering and amending income-tax brackets from the new
  screen — including the simulator that proves the calculation before
  payroll runs, and the validation refusing a set with a gap between
  brackets — plus the revenue tax profiles screen.
- The voucher account lookup now scoped per side, shown for both a
  صرف and a قبض voucher, and the "عرض التفاصيل" drill-down on the
  accounting gaps screen listing the real documents behind each count.