docs(deep-sleep-stub.rst): Added documentation about re-entering deep sleep from wake stub (IDFGH-12569)

[Lupindakaas]

This PR adds documentation to the deep sleep wake stub docs detailing how to re-enter deep sleep from the wake stub.

[github-actions]

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[esp-wzh]

REG_SET_FIELD(RTC_CNTL_STATE0_REG, RTC_CNTL_ULP_CP_SLP_TIMER_EN, 1) is for enabling the timer wake-up source for the ULP coprocessor, which is independent of the deep-sleep-stub
feature is not a universal usage method, I think it is not necessary to explain it.

[esp-wzh]

@Lupindakaas
Thank you for pointing out this flaw in the IDF deepsleep stub example! The configuration of the wake-up source in the example is hidden in the call to the esp_sleep_enable_timer_wakeup function and sleep process in the main core app main, and then subsequent wake-ups in the stub inherit this enablement. If the wake-up source needs to be changed while the stub is running, other users is hard to obtain relevant information.

But the description in this file should be applicable to all esp chips. For some other esp32 series chips, the registers name and function have changed. Do you mind if I restart an update to this doc internally?

[Lupindakaas]

Hi, that would be awesome! I also send an email on the feedback form with some more documentation is missed whist working on our wake stub code, maybe you could check that out too.