Finetune requirements and documentation of using client credentials with openeo

[soxofaan]

With #624 a new doc page was added explaining some CDSE openEO authentication details, including the feature of using the client credentials created from the SentinelHub dashboard app:
https://documentation.dataspace.copernicus.eu/APIs/openEO/authentication.html#obtaining-client-credentials

documentation/APIs/openEO/authentication.qmd

Lines 195 to 200 in 7dce3b4

	The Sentinel Hub service in the Copernicus Data Space Ecosystem
	has a [dashboard web app](https://shapps.dataspace.copernicus.eu/dashboard)
	and under the account settings there is a self-service feature to
	[register your own OAuth client](../SentinelHub/Overview/Authentication.qmd#registering-oauth-client){target="_blank"}.
	The client id and client secret obtained here can also be used
	for the client credentials flow with the openEO service of Copernicus Data Space Ecosystem.

This section is intentionally a bit vague at the moment, and needs some more finetuning.

For example from @rahmandawibowo-vito I learned that it's not immediately possible to "link" the client credentials with a user account, the user first has to run a batch job with these client credentials, so that an account for this client is created in the marketplace.

What also needs some clarification I think is how credits are associated, how they are linked (or not) between user and client, and how it is represented in the marketplace.

cc @rahmandawibowo-vito @JanssenBrm

[soxofaan]

FYI: with #636 I added this section to the CDSE docs on how to get the OIDC "sub" identifier of a service account: https://documentation.dataspace.copernicus.eu/APIs/openEO/authentication/client_credentials.html#obtain-the-oidc-identifier-of-a-service-account

[soxofaan]

related (internal) cross-reference: MKTP-471