doc: ieee802154: improved API documentation structure

Improves the API documentation including it's structure and integration
into the Zephyr documentation.

Previously the API status of the IEEE 802.15.4 API (and its sub-APIs)
was unclear.

This change adds three APIs in an "unstable" state:
- IEEE 802.15.4 L2 for subsystem developers
- IEEE 802.15.4 drivers for driver developers
- IEEE 802.15.4 net_mgmt for application developers

The corresponding APIs are documented at a group level.

These sub-APIs need to be separately versioned as they have different
audiences, change velocities and levels of stability.

Signed-off-by: Florian Grandel <fgrandel@code-for-humans.de>