doc: power management: Remove references to previous PM states terminology

[erwango]

Describe the bug

Following merge of #31273
https://docs.zephyrproject.org/latest/reference/power_management/index.html was not updated.
Particularly the following terminology is not up to date anymore:

:dfn:`Active State`
   The CPU and clocks are powered on. This is the normal operating state when
   the system is running.

:dfn:`Sleep State`
   Some of the SoC clocks are gated. The CPU is stopped but does not lose
   execution context. Configuration of the peripherals is preserved.

:dfn:`Deep Sleep State`
   The SoC is power gated and loses context. Most peripherals would also be
   power gated. RAM may be selectively retained.

:dfn:`Idle Thread`
   A system thread that runs when there are no other threads ready to run.

:dfn:`Power gating`
   Power gating reduces power consumption by shutting off current to blocks of
   the integrated circuit that are not in use.

To Reproduce
See doc.

Impact
Confusion

[nashif]

@ceolin is working on that already, lets have just 1 assignee and avoid 2 people working on the same thing.