Add documentation for the serialization of transactions

[orhoj]

Update the documentation to include descriptions of the serialization of transactions that are not already described. I suggest that it is done similarly as in https://github.com/Concordium/concordium-node/blob/main/docs/grpc-for-smart-contracts.md. This documentation is needed as external parties are interested in how to serialize the different transactions, and currently we refer them to the implementation of that serialization in the Haskell code.

Consider limiting the scope to account transactions and credential deployment as the update transactions are less likely to be needed elsewhere at this time.

[abizjak]

I think it would be better if this was gathered in developer documentation (so there is one place to go get the documentation) rather than in docs folders in specific repositories.

The only reason the smart contracts one is there is that it was done in a rush.