Add docs with sequence diagrams for operations

docs/sequence.md

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+# Slurm Appliance Sequences
+
+
+
+## Image build
+
+This sequence applies to both "fatimage" builds (usually only done in StackHPC CI) and "extra" builds. The differences are:
+- Which image the build VM uses, i.e. the starting image: A genericcloud image for fatimage builds or a fatimages build for an extra build.
+- Which inventory groups the build VM is added to.
+
+Note that ansible-init does not run during an image build. It is disabled via a metadata flag.
+
+```mermaid
+sequenceDiagram
+    participant ansible as Ansible Deploy Host
+    participant cloud as Cloud
+    note over ansible: $ packer build ...
+    ansible->>cloud: Create VM
+    create participant packer as Build VM
+    cloud->>packer: Create VM
+    note over packer: Boot
+    ansible->>packer: Wait for ssh connection
+    ansible->>packer: Run ansible/fatimage.yml playbook
+    ansible->>packer: Shutdown
+    ansible->>cloud: Create image from Build VM root disk
+    destroy packer
+    note over cloud: Image openhpc-... created
+
+```
+
+## Cluster Creation
+
+```mermaid
+sequenceDiagram
+    participant ansible as Ansible Deploy Host
+    participant cloud as Cloud
+    note over ansible: $ ansible-playbook ansible/adhoc/generate-passwords.yml
+    ansible->>ansible: Template secrets to inventory group_vars
+    note over ansible: $ tofu apply ....
+    ansible->>cloud: Create infra
+    create participant nodes as Cluster Instances
+    cloud->>nodes: Create instances
+    note over nodes: Boot
+    rect rgb(204, 232, 252)
+    note over nodes: ansible-init
+    nodes->>cloud: Query metadata
+    cloud->>nodes: Metadata sent
+    nodes->>nodes: Start k3s
+    end
+    note over ansible: $ ansible-playbook ansible/site.yml
+    ansible->>nodes: Wait for ansible-init completion
+    ansible->>nodes: Ansible tasks
+    note over nodes: All services running
+