Documentation of federated credit usage

[Pratichhya]

Currently, we have this as the information source for federated credits and their usage. However when comparing the actual usage it could be a bit confusing:

https://documentation.dataspace.copernicus.eu/APIs/openEO/federation/openeo_federation.html#federation-credit-usage

Maybe we can try to improve and simplify it also in https://documentation.dataspace.copernicus.eu/APIs/openEO/credit_usage.html section