Simplify the documentation around message payload encoding

Currently the doc is explaining a sort of encoding with key values pairs

The keys are encoded as a variable length integer tag where the 3 lowest bits indicates the type of the value: 0 for integers, 2 for strings....

And talking about tags 0x0A, 0x10, 0x22

Given that it is just plain protobuf, like 0xA is 0| 0001 | 010 that is tag 1 and type 2 (bytes).

Probably we could just simplify and put the proto definitions:

package messages;
syntax = "proto3";

message PreKey {
    bytes one_time_key = 1;
    bytes base_key = 2;
    bytes identity_key = 3;
    bytes message = 4;
}

message Normal {
    bytes ratchet_key = 1;
    uint64 chain_index = 2;
    bytes cipher_text = 4;
}