hardware: refactor tether interfaces to be a recommended extra

[ES-Alexander]

Currently our required/recommended hardware diagram shows fathom-Xs, but we don't have a meaningful overview description of what they are/what they do. The details are in the Tethers page, but it seems like it would be more appropriate to have in a separate page of its own, as part of the recommended extras.

[ES-Alexander]

This is extra relevant because they're not strictly required for use with an ethernet-based tether, and not at all relevant for a fiber-optic tether (as discussed here), which should instead use a different kind of interface board (e.g. Ethernet to Fiber Optic).