chore(docs): add guide how to create service account in empty org (#862)

relates to #855

docs/guides/stackit_org_service_account.md

@@ -0,0 +1,15 @@
+---
+page_title: "Creating projects in empty organization via Terraform"
+---
+# Creating projects in empty organization via Terraform
+
+Consider the following situation: You're starting with an empty STACKIT organization and want to create projects 
+in this organization using the `stackit_resourcemanager_project` resource. Unfortunately it's not possible to create
+a service account on organization level which can be used for authentication in the STACKIT Terraform provider. 
+The following steps will help you to get started:
+
+1. Using the STACKIT portal, create a dummy project in your organization which will hold your service account, let's name it e.g. "dummy-service-account-project".
+2. In this "dummy-service-account-project", create a service account. Create and save a service account key to use for authentication for the STACKIT Terraform provider later as described in the docs. Now copy the e-mail address of the service account you just created.
+3. Here comes the important part: Navigate to your organization, open it and select "Access". Click on the "Grant access" button and paste the e-mail address of your service account. Be careful to grant the service account enough permissions to create projects in your organization, e.g. by assigning the "owner" role to it.
+
+*This problem was brought up initially in [this](https://github.com/stackitcloud/terraform-provider-stackit/issues/855) issue on GitHub.*

docs/resources/resourcemanager_project.md

@@ -4,12 +4,15 @@ page_title: "stackit_resourcemanager_project Resource - stackit"
 subcategory: ""
 description: |-
   Resource Manager project resource schema. To use this resource, it is required that you set the service account email in the provider configuration.
+  -> In case you're getting started with an empty STACKIT organization and want to use this resource to create projects in it, check out this guide https://registry.terraform.io/providers/stackitcloud/stackit/latest/docs/guides/stackit_org_service_account for how to create a service account which you can use for authentication in the STACKIT Terraform provider.
 ---
 
 # stackit_resourcemanager_project (Resource)
 
 Resource Manager project resource schema. To use this resource, it is required that you set the service account email in the provider configuration.
 
+-> In case you're getting started with an empty STACKIT organization and want to use this resource to create projects in it, check out [this guide](https://registry.terraform.io/providers/stackitcloud/stackit/latest/docs/guides/stackit_org_service_account) for how to create a service account which you can use for authentication in the STACKIT Terraform provider.
+
 ## Example Usage
 
 ```terraform

stackit/internal/services/resourcemanager/project/resource.go

@@ -91,7 +91,10 @@ func (r *projectResource) Configure(ctx context.Context, req resource.ConfigureR
 // Schema defines the schema for the resource.
 func (r *projectResource) Schema(_ context.Context, _ resource.SchemaRequest, resp *resource.SchemaResponse) {
 	descriptions := map[string]string{
-		"main":                "Resource Manager project resource schema. To use this resource, it is required that you set the service account email in the provider configuration.",
+		"main": fmt.Sprintf("%s\n\n%s",
+			"Resource Manager project resource schema. To use this resource, it is required that you set the service account email in the provider configuration.",
+			"-> In case you're getting started with an empty STACKIT organization and want to use this resource to create projects in it, check out [this guide](https://registry.terraform.io/providers/stackitcloud/stackit/latest/docs/guides/stackit_org_service_account) for how to create a service account which you can use for authentication in the STACKIT Terraform provider.",
+		),
 		"id":                  "Terraform's internal resource ID. It is structured as \"`container_id`\".",
 		"project_id":          "Project UUID identifier. This is the ID that can be used in most of the other resources to identify the project.",
 		"container_id":        "Project container ID. Globally unique, user-friendly identifier.",

templates/guides/stackit_org_service_account.md.tmpl

@@ -0,0 +1,15 @@
+---
+page_title: "Creating projects in empty organization via Terraform"
+---
+# Creating projects in empty organization via Terraform
+
+Consider the following situation: You're starting with an empty STACKIT organization and want to create projects 
+in this organization using the `stackit_resourcemanager_project` resource. Unfortunately it's not possible to create
+a service account on organization level which can be used for authentication in the STACKIT Terraform provider. 
+The following steps will help you to get started:
+
+1. Using the STACKIT portal, create a dummy project in your organization which will hold your service account, let's name it e.g. "dummy-service-account-project".
+2. In this "dummy-service-account-project", create a service account. Create and save a service account key to use for authentication for the STACKIT Terraform provider later as described in the docs. Now copy the e-mail address of the service account you just created.
+3. Here comes the important part: Navigate to your organization, open it and select "Access". Click on the "Grant access" button and paste the e-mail address of your service account. Be careful to grant the service account enough permissions to create projects in your organization, e.g. by assigning the "owner" role to it.
+
+*This problem was brought up initially in [this](https://github.com/stackitcloud/terraform-provider-stackit/issues/855) issue on GitHub.*