docs: Networking with native_posix: Rename

Rename
Networking with native_posix
to
Networking with native_sim
replace all links to this page accordingly
and replace its references to native_posix with native_sim.

Background: during this release native_sim is replacing
native_posix as the main host test/development platform.

Signed-off-by: Alberto Escolar Piedras <alberto.escolar.piedras@nordicsemi.no>