Rename argus to observability (#510)

* Create observability resources

* add deprecation message to argus

* add warning to readme

* remove warning from readme

* add info back to migration md

* Update MIGRATION.md

Co-authored-by: João Palet <joao.palet@outlook.com>

---------

Co-authored-by: João Palet <joao.palet@outlook.com>

docs/resources/observability_credential.md

@@ -0,0 +1,34 @@
+---
+# generated by https://github.com/hashicorp/terraform-plugin-docs
+page_title: "stackit_observability_credential Resource - stackit"
+subcategory: ""
+description: |-
+  Observability credential resource schema. Must have a region specified in the provider configuration.
+---
+
+# stackit_observability_credential (Resource)
+
+Observability credential resource schema. Must have a `region` specified in the provider configuration.
+
+## Example Usage
+
+```terraform
+resource "stackit_observability_credential" "example" {
+  project_id  = "xxxxxxxx-xxxx-xxxx-xxxx-xxxxxxxxxxxx"
+  instance_id = "xxxxxxxx-xxxx-xxxx-xxxx-xxxxxxxxxxxx"
+}
+```
+
+<!-- schema generated by tfplugindocs -->
+## Schema
+
+### Required
+
+- `instance_id` (String) The Observability Instance ID the credential belongs to.
+- `project_id` (String) STACKIT project ID to which the credential is associated.
+
+### Read-Only
+
+- `id` (String) Terraform's internal resource ID. It is structured as "`project_id`,`instance_id`,`username`".
+- `password` (String, Sensitive) Credential password
+- `username` (String) Credential username