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 <hebad@vestas.com>