doc: can: move the documention for high-level protocols to connectivity

Move the documentation for high-level CAN protocols (for now only covering
ISO-TP) from the peripherals section to the connectivity section.

This matches the layout in code, where the CAN controllers are under the
drivers/can directory and the protocols are under the subsys/canbus/
directory.

Signed-off-by: Henrik Brix Andersen <hebad@vestas.com>