Document the CheeTah T3 network protocol

[uellue]

Currently the implicit documentation is in https://github.com/LiberTEM/LiberTEM-rs/blob/main/libertem_asi_tpx3/src/headers.rs and https://github.com/LiberTEM/LiberTEM-rs/blob/main/libertem_asi_tpx3/src/receiver.rs. In the future we should have this documented properly in human-readable form, possibly with corresponding test cases.