Update docs to choose reference over pointer

[hamishwillee]

As discussed here: mavlink/MAVSDK#314. Depends on this PR: mavlink/MAVSDK#316

From a docs perspective this pretty much involves doing a global search->replace of "&device" to device where the device is used:

auto telemetry = std::make_shared<Telemetry>(&device);

TO

auto telemetry = std::make_shared<Telemetry>(device);

BUT cannot do a pure replace, because we still create the reference in the same way:

Device &device = dc.device();

Will also need to rebuild API reference.

[hamishwillee]

Will be fixed by #107