doc: doxygen: add some subsystems to os_services group

Add the following root Doxygen modules under the umbrella of "OS
Services" Doxygen group:

- Cache
- File system
- IPC
- RTIO
- CRC
- Iterable sections
- ZBus
- Task watchdog
- Shell
- Semihosting
- Logging
- System init

Signed-off-by: Gerard Marull-Paretas <gerard.marull@nordicsemi.no>