Update the documentation for OID4VCI Authorization Flow 1a

[yshyn-iohk]

Update the doc with the corresponding changes in the cloud-agent and SDKs

• add the general documentation
• add the tutorial
• add the configuration guide for the Keycloak and Credential Issuer (if needed)

[yshyn-iohk]

Moving the issue back to the cloud-agent repository.
We will pick it up later when it is necessary.