Add ServerMaintenance API type

docs/concepts/servermaintenance.md

@@ -0,0 +1,76 @@
+# ServerMaintenance
+
+`ServerMaintenance` represents a maintenance operation for a physical server. It transitions a `Server` from its 
+current operational state (e.g., Available/Reserved) into a Maintenance state. Each `ServerMaintenance` object tracks the lifecycle of a maintenance task, ensuring servers are properly taken offline, updated, and restored.
+
+## Key Points
+
+- `ServerMaintenance` is namespaced and may represent various maintenance operations.
+- Only one `ServerMaintenance` can be active per `Server` at a time. Others remain pending.
+- When the active `ServerMaintenance` completes, the next pending one (if any) starts.
+- If no more maintenance tasks are pending, the `Server` returns to its previous operational state.
+- `policy` determines how maintenance starts:
+    - **OwnerApproval:** Requires a label (e.g., `ok-to-maintenance: "true"`) on the `ServerClaim`.
+    - **Forceful:** Does not require owner approval.
+
+## Workflow
+
+1. A separate operator (e.g., `foo-maintenance-operator`) or user creates a `ServerMaintenance` resource referencing a 
+   specific `Server`.
+2. If `policy` is `OwnerApproval` and no `ok-to-maintenance` label is set on the `ServerClaim`, the `ServerMaintenance`
+   stays in `Pending`. The `Server` also remains unchanged.
+3. If `policy` is `OwnerApproval` and the `ok-to-maintenance` label is present (or if `alwaysPerformMaintenance` is 
+   enabled), or if the policy is `Forceful`, the `metal-operator` transitions the `Server` into `Maintenance` and 
+   updates the `ServerMaintenance` state accordingly.
+4. The maintenance operator powers off the `Server`, applies a `ServerBootConfiguration` (if needed), performs the 
+   maintenance, and sets `ServerMaintenance` to `Complete`.
+5. The `metal-operator` transitions the `Server` back to its prior state. If additional `ServerMaintenance` objects are
+   pending, the next one is processed.
+
+## Sequence Diagram
+
+```mermaid
+sequenceDiagram
+    participant Ext as External entity (e.g. foo-maintenance-operator)
+    participant SC as ServerClaim
+    participant MO as metal-operator
+    participant SM as ServerMaintenance
+    participant S as Server
+    participant SBC as ServerBootConfiguration
+
+    Ext->>MO: Create SM (ServerMaintenance) resource for S
+    MO->>SC: Check for ok-to-maintenance label (if policy=OwnerApproval)
+    alt Policy = OwnerApproval AND no ok-to-maintenance label
+        Note over MO: SM stays Pending, Server remains unchanged
+    else Policy = OwnerApproval AND label present OR Policy = Forceful
+        MO->>S: Update Server state to "InMaintenance"
+        MO->>SM: Set SM state to "InMaintenance"
+        Ext->>S: Power off Server
+        Ext->>MO: Create SBC and reference in Server
+        Note over Ext: Perform maintenance
+        Ext->>SM: Update SM state to "Complete"
+        MO->>S: Return Server to Available/Reserved
+        Ext->>S: Power on Server
+    end
+```
+
+## Example
+
+```yaml
+apiVersion: metal.ironcore.dev/v1alpha1
+kind: ServerMaintenance
+metadata:
+  name: bios-update
+  namespace: ops
+spec:
+  policy: OwnerApproval
+  serverRef:
+    name: server-foo
+status:
+  state: Pending
+```
+
+If `policy: OwnerApproval` and no `ok-to-maintenance` label exists on the `ServerClaim`, this `ServerMaintenance` 
+remains `Pending`, and the `Server` stays as is. Once the label is added (or if the operator setting 
+`alwaysPerformMaintenance` is enabled), the `metal-operator` transitions the `Server` to `InMaintenance`, and the 
+maintenance operator performs the maintenance task.

mkdocs.yml

@@ -56,6 +56,7 @@ nav:
     - Servers: concepts/servers.md
     - ServerBootConfigurations: concepts/serverbootconfigurations.md
     - ServerClaims: concepts/serverclaims.md
+    - ServerMaintenance: concepts/servermaintenance.md
 - Usage:
   - metalctl: usage/metalctl.md
 - Development Guide: