doc: reference: overview: bump EEPROM API to unstable
doc: reference: overview: bump EEPROM API to unstable

The EEPROM API, which was introduced in Zephyr v2.1.0 and has not seen
any changes since, has multiple implementations supporting a wide
variety of EEPROM backends (SPI, I2C, on-chip, simulator).

Bump the EEPROM API from "experimental" to "unstable" according to the
Zephyr API lifecycle process.

Signed-off-by: Henrik Brix Andersen <24a3f13c22635bab658e39fc5f1e63c575328aae@vestas.com>

galak/zephyr,galak/zephyr,Vudentz/zephyr,galak/zephyr,finikorg/zephyr,galak/zephyr,Vudentz/zephyr,nashif/zephyr,finikorg/zephyr,zephyrproject-rtos/zephyr,finikorg/zephyr,zephyrproject-rtos/zephyr,nashif/zephyr,galak/zephyr,finikorg/zephyr,Vudentz/zephyr,zephyrproject-rtos/zephyr,nashif/zephyr,zephyrproject-rtos/zephyr,nashif/zephyr,Vudentz/zephyr,nashif/zephyr,finikorg/zephyr,zephyrproject-rtos/zephyr,Vudentz/zephyr,Vudentz/zephyr