doc: openthread: integrate ot/ieee802154 docs

Integrates and cross-references OpenThread and native IEEE 802.15.4 docs
as they share a common driver layer.

While OpenThread is of course not an IEEE 802.15.4 subsystem API it is
still included in the ieee802154 documentation group, purely for
convenience to the documentation readers.

For organizational purposes OT is considered a separate area of
maintenance and therefore keeps its dedicated documentation page - but
the two subsystems now cross-reference each other for convenience.

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