Allow `Device` types to link to one another

[rly]

Related to https://github.com/NeurodataWithoutBorders/pynwb/issues/1250

An experimental session often involves many devices, but the current schema usually allows for linking of only a single device to a time series and devices cannot link to one another. Users can make extensions that create this link but it would be useful to

1. build into NWB a link from a generic Device to related Devices and
2. add some of the common Device types into NWB, such as Probe, Headstage, DataAcquisitionSystem, Camera, etc. Adding a Probe type was previously discussed. See https://github.com/NeurodataWithoutBorders/nwb-schema/issues/329 and https://github.com/rly/ndx-probe