Populate / extend the requirements documents with Timer requirements

• analyze existing requirements in the Zephyr System Requirements and Software Requirements
  strictDoc files regarding completeness to the associated docs (https://docs.zephyrproject.org/latest/kernel/services/timing/timers.html) and source files timer.c
• add the requirements to the section/file associated with Timers in a sdoc file, if no section or file exists, add new section (https://github.com/zephyrproject-rtos/reqmgmt/tree/main/docs/system_requirements) or file (https://github.com/zephyrproject-rtos/reqmgmt/tree/main/docs/software_requirements)
• establish linking between requirements (Software to System Requirements), using sdoc requirements linking functionality