[doc issue] Terraforming Kyma Runtimes - Prerequistes technical user setup

[lechnerc77]

Issue description

The prerequisites are unclear and I would say more a recommendation:

- The development team must have admin access at the subaccount level or SAP BTP global account level.
- The development team must have its own [Identity Authentication tenant](https://help.sap.com/docs/cloud-identity-services/cloud-identity-services/get-your-tenant).
- The development team must create a technical user.
- The technical user must be invited to the custom Identity Authentication tenant.

It should be more consise that we recommend to have technical user in place for the CI/CD pipeline. It is required that the technical user is assigned the role collection"Subaccount Administrator" for the module to work.

Feedback Type (Optional)

incorrect statement

Page Title on SAP Help Portal (prefilled)

Terraforming Kyma Runtimes

Page URL on SAP Help Portal (prefilled)

https://help.sap.com/docs/btp/btp-developers-guide/terraforming-kyma-runtimes?version=LATEST

[kwiatekus]

### Prerequisites
 - Terraform CLI installed on the runner. See: Install Terraform and the Terraform CLI.
 - Administrative access to an SAP BTP global account or target subaccount
 
### Recommendations for  CI/CD Scenario

Avoid personal identities. Use a technical user configured via a dedicated Identity Authentication (IAS) tenant.

Steps:

 - Obtain an IAS tenant and configure trust to SAP BTP (global account or subaccount scope).
 - Request a shared mailbox to serve as the technical user’s username.
 - Invite the technical user to IAS, set the password, store it securely (vault/secret manager).
 - Assign the technical user the required role collections in SAP BTP (global account admin or subaccount admin depending on scope).

Access Models (scopes)
1) Reuse Existing Subaccount (Team has only subaccount admin rights):
 - Trust IAS at subaccount level.
 - Assign subaccount admin role collection to technical user.
 - Module operates within the existing subaccount (`BTP_USE_SUBACCOUNT_ID` set).
 Example: kyma-on-btp-reuse-sa.

2) Create New Subaccount (Team has global account admin rights):
 - Trust IAS at global account level.
 - Assign global account admin role collection to technical user.
 - Module creates and manages a new subaccount plus Kyma runtime.
 Example: kyma-on-btp-new-sa.