Add peripheral documentation gap analysis

[Copilot]

Identifies peripheral driver classes missing or with incomplete documentation in doc/hardware/peripherals/ by comparing against headers in include/zephyr/drivers/.

Findings

21 peripherals with no documentation:
cellular, disk, espi_saf, ethernet, firmware, fpga, ieee802154, interrupt_controller, led_strip, memc, mfd, mic_privacy, mipi_dbi, mipi_dsi, modem, pm_cpu_ops, power, sip_svc, swdp, syscon, tee

8 peripherals with stub docs (API ref only, ~13 lines):
adc, counter, i2c_eeprom_target, ipm, pcie, pwm, spi, watchdog

11 peripherals with incomplete docs (<25 lines):
bbram, clock_control, crc, dac, entropy, hwspinlock, mbox, mdio, psi5, sent, tgpio

Document also notes peripherals documented elsewhere (bluetooth→connectivity/bluetooth/, usb→connectivity/usb/, etc.) and references well-documented peripherals like gpio.rst (153 lines) as templates.

Original prompt

I want the list of all peripheral classes ("driver") that are missing docs in hardware/
for example GPIO has docs, dac has but it's very incomplete, etc
use headers in include/drivers to get a sense of all the peripherals that exist

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