feat(observability): add description to credential resource (#992)

relates to STACKITTPR-333, #974

docs/resources/observability_credential.md

@@ -16,6 +16,7 @@ Observability credential resource schema. Must have a `region` specified in the
 resource "stackit_observability_credential" "example" {
   project_id  = "xxxxxxxx-xxxx-xxxx-xxxx-xxxxxxxxxxxx"
   instance_id = "xxxxxxxx-xxxx-xxxx-xxxx-xxxxxxxxxxxx"
+  description = "Description of the credential."
 }
 ```
 
@@ -27,6 +28,10 @@ resource "stackit_observability_credential" "example" {
 - `instance_id` (String) The Observability Instance ID the credential belongs to.
 - `project_id` (String) STACKIT project ID to which the credential is associated.
 
+### Optional
+
+- `description` (String) A description of the credential.
+
 ### Read-Only
 
 - `id` (String) Terraform's internal resource ID. It is structured as "`project_id`,`instance_id`,`username`".