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).

Start with the Transaction Fees documentation section linked in the issue and review the Euro2CCD oracle service repository. Identify the service’s system design and potential error codes from those sources, then expand the documentation so the oracle is explained in depth and clearly linked from the transaction-cost section.

Written by the indexing model from the issue text.