docs: native_sim/posix: Swap documentation from native_posix to native_sim

During this release native_sim is replacing native_posix
as the main host test/development platform.

Therefore, instead of basing the native_sim documentation
in native_posix's one, let's do it the other way around.

This commit mostly moves documentation from the native_posix
page into the native_sim one, changing the board and renaming
labels.

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