docs: update README and postman collection

[paullatzelsperger]

Description

this PR updates the README.md to reflect the recent changes related to IdentityHub and the CatalogServer.

In addition, it adds several Requests to the Postman collection to communicate with the IdentityHub APIs.

Open TODOs

• The Postman collection only contains select requests to walk through a typical contract+datatransfer sequence. If the entirety of EDC Management API requests is needed, they'd need to be added subsequently.
• The section about adding a third participant was removed, because the current deployment doesn't contain related code anymore.
• The section about deploying MXD to a third-party K8S cluster will likely need to be updated. I didn't check.

Other stuff

updated Gradle to 8.10

Pre-review checks

Please ensure to do as many of the following checks as possible, before asking for committer review:

• DEPENDENCIES are up-to-date. Dash license tool. Committers can open IP issues for restricted libs.
• Copyright and license header are present on all affected files