ref 624723: server backup schedules (#416)

Signed-off-by: Adrian Nackov <adrian.nackov@mail.schwarz>

docs/resources/server_backup_schedule.md

@@ -0,0 +1,60 @@
+---
+# generated by https://github.com/hashicorp/terraform-plugin-docs
+page_title: "stackit_server_backup_schedule Resource - stackit"
+subcategory: ""
+description: |-
+  Server backup schedule resource schema. Must have a region specified in the provider configuration.
+---
+
+# stackit_server_backup_schedule (Resource)
+
+Server backup schedule resource schema. Must have a `region` specified in the provider configuration.
+
+## Example Usage
+
+```terraform
+resource "stackit_server_backup_schedule" "example" {
+  project_id = "xxxxxxxx-xxxx-xxxx-xxxx-xxxxxxxxxxxx"
+  server_id  = "xxxxxxxx-xxxx-xxxx-xxxx-xxxxxxxxxxxx"
+  name       = "example_backup_schedule_name"
+  rrule      = "DTSTART;TZID=Europe/Sofia:20200803T023000 RRULE:FREQ=DAILY;INTERVAL=1"
+  enabled    = true
+  backup_properties = {
+    name             = "example_backup_name"
+    retention_period = 14
+    volume_ids       = null
+  }
+}
+```
+
+<!-- schema generated by tfplugindocs -->
+## Schema
+
+### Required
+
+- `enabled` (Boolean) Is the backup schedule enabled or disabled.
+- `name` (String) The schedule name.
+- `project_id` (String) STACKIT Project ID to which the server is associated.
+- `rrule` (String) Backup schedule described in `rrule` (recurrence rule) format.
+- `server_id` (String) Server ID for the backup schedule.
+
+### Optional
+
+- `backup_properties` (Attributes) Backup schedule details for the backups. (see [below for nested schema](#nestedatt--backup_properties))
+
+### Read-Only
+
+- `backup_schedule_id` (Number) Backup schedule ID.
+- `id` (String) Terraform's internal resource identifier. It is structured as "`project_id`,`server_id`,`backup_schedule_id`".
+
+<a id="nestedatt--backup_properties"></a>
+### Nested Schema for `backup_properties`
+
+Required:
+
+- `name` (String)
+- `retention_period` (Number)
+
+Optional:
+
+- `volume_ids` (List of String)