ArduSub: link to DataFlash .bin format spec

[ES-Alexander]

I went looking for some docs for this, and apparently it's "self describing".

This forum comment was the best attempt at an explanation I could find, and pretty much everything else just recommended looking at implementations (like Pymavlink's DFReader.py).

The "adding a message" link that is already included in our docs describes the parameters for ArduPilot's AP::logger().Write method, which roughly describes what should end up in the file, but not how it's packed in as bits to the binary logs (which makes sense since AP::logger is generic to multiple logging backends).

It would be good if we could have a more cohesive place to link to that describes the file format, even if that means writing the relevant docs for ArduPilot and then linking to them.