doc: hidden path design update (#3900)

This adapts the original proposal to changes we made to the rest of the
SCION ecosystem:

- No longer speaks of PS/BS etc.
- Uses service discovery instead of a separate SVC address.
- Services are defined in gRPC.
- Configs use snake_case.

Apart from that small functional adaptions are proposed:

- The SCION daemon is configured with the hidden path group IDs it
  should query for, instead of having this in the path lookup request.
  This makes everything a bit simpler, doesn't need a special lookup for
  the hidden path config etc.

[doc]