Document EURO/CCD oracle

**Description**

[Euro2CCD oracle service](https://github.com/Concordium/concordium-euro2ccd-service) should be described more in-depth. 

It is briefly mentioned in the documentation on [Transaction Fees](https://docs.concordium.com/en/mainnet/docs/protocol/transaction-fees.html#computing-transaction-costs), but not explained in full (e.g. with potential error codes or system design). 