DRTIO switching documentation

[hartytp]

Please can you update https://m-labs.hk/artiq/manual-master/drtio.html?highlight=drtio and https://m-labs.hk/artiq/manual-master/installing.html to include some basic info about switching, point people to artiq_route etc.

(Should this also at least mention SED?)

[hartytp]

Documentation in artiq_route could be a bit clearer.

• we're assigning numerical identifiers to each DRTIO satellite and then specifying the route from the master to the satellite as a series of hops
• are a number with 0 for local RTIO and n for the slave device connected to sfp n+1 (taking care over zero-indexing)
• routes must end in 0
• as with standard DRTIO, rtio channels on slaves are accessed by: bits 0-15 are the local channel number, starting afresh from 0, for each slave; bits 16-24 are the destination number

[hartytp]

no special gateware/firwmare configuration is required, since all satellites are now DRTIO repeaters by default

[hartytp]

NB route 0 0 does need to be explicitly programmed otherwise local RTIO channels aren't available

[hartytp]

if no routing table is programmed a default one is adopted