init_levels build target should be documented

[kartben]

Is your enhancement proposal related to a problem? Please describe.

The newly introduced initlevels build target (#62459) is a pretty cool tool, but it's not documented (although to be 100% complete it is mentioned in the 3.5 release notes) and people could easily miss it.

Describe the solution you'd like

Add documentation about the target and how it can be used somewhere in docs. Maybe https://docs.zephyrproject.org/latest/develop/west/build-flash-debug.html, but would probably be nice to also mention it as a useful troubleshooting tool in e..g https://docs.zephyrproject.org/latest/kernel/drivers/index.html#initialization-levels

Describe alternatives you've considered

N/A

Additional context

N/A