Documentation updates: DJ pdf and devices

Solaar output added for M525 and K750, add T650 reprogrammable keys (after bugfix).

DJ spec is also added (likely not useful for Solaar, but its ideas may give a better understanding of the HID++ concepts).

Note: the K750 does not show battery information in CLI. I guess this happens because the battery status for `SOLAR_DASHBOARD` is not immediately returned (it takes a second). Possible solutions:

- hard code a `sleep(1)`
- introduce ability to wait for a notification or property update (with timeout to prevent hang)
- ignore this issue, people are mostly not interested in battery updates via CLI since it is slow (because of feature/keys enumeration). Use UPower or Solaar GUI instead.