init: improve Doxygen

- Create a sys_init entry, as init API is not strictly related to the
  device model (device model re-uses it)
- Hide internals (Z_*) from public API docs.
- Give overview within the group definition of allowed levels, meaning,
  etc.
- Properly document struct init_entry.
- Multiple style issues

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