docs: create a flow diagram of the Ignition services

docs/ignition-flow.md

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+```mermaid
+flowchart TB
+    %% ===== IGNITION BOOT FLOW =====
+
+    %% --- Early Boot ---
+    boot["Boot"] --> setup_pre["ignition-setup-pre.service"]
+    setup_pre --> setup["ignition-setup.service"]
+    setup --> fetch_offline["ignition-fetch-offline.service"]
+
+    %% --- Fetch Offline Details ---
+    subgraph FETCH_OFFLINE ["Ignition Fetch Offline"]
+        direction TB
+        offline_detect_platform["Detect platform"]
+        offline_check_configs["Check configs at:"]
+        offline_base_dir["/usr/lib/ignition/base.d"]
+        offline_platform_dir["/usr/lib/ignition/base.platform.d/{platform}"]
+        offline_detect_platform --> offline_check_configs
+        offline_check_configs --> offline_base_dir
+        offline_check_configs --> offline_platform_dir
+        offline_merge_configs["Merge configs if present"]
+        offline_base_dir --> offline_merge_configs
+        offline_platform_dir --> offline_merge_configs
+    end
+    fetch_offline --> FETCH_OFFLINE
+
+    FETCH_OFFLINE --> fetch_service["ignition-fetch.service"]
+
+    %% --- Fetch Service Details ---
+    subgraph FETCH_ONLINE ["Ignition Fetch"]
+        direction TB
+        online_detect_platform["Detect platform"]
+        online_check_configs["Check configs at:"]
+        online_base_dir["/usr/lib/ignition/base.d"]
+        online_platform_dir["/usr/lib/ignition/base.platform.d/{platform}"]
+        online_detect_platform --> online_check_configs
+        online_check_configs --> online_base_dir
+        online_check_configs --> online_platform_dir
+        online_request_cloud_configs["Request cloud specific configs"]
+        online_cloud_configs_present{"Cloud configs present?"}
+        online_use_cloud_configs["Merge configs if present"]
+        online_open_config_device["Open and read config device"]
+        online_base_dir --> online_request_cloud_configs
+        online_platform_dir --> online_request_cloud_configs
+        online_request_cloud_configs --> online_cloud_configs_present
+        online_cloud_configs_present -->|Yes| online_use_cloud_configs
+        online_cloud_configs_present -->|No| online_open_config_device
+        online_open_config_device --> online_use_cloud_configs
+    end
+    fetch_service --> FETCH_ONLINE
+
+    %% --- Network Stack ---
+    subgraph NETWORK ["Network Stack"]
+        direction TB
+        networkd_service["systemd-networkd.service"]
+        find_primary_nic["Find primary NIC"]
+        link_up["Link up"]
+        network_config["systemd-networkd.service - Network Configuration"]
+        network_target["network.target reached"]
+        networkd_service --> find_primary_nic --> link_up --> network_config --> network_target
+    end
+    setup --> NETWORK
+    NETWORK --> FETCH_ONLINE
+    NETWORK --> get_dhcp_address["Get DHCP address"]
+    get_dhcp_address --> online_request_cloud_configs
+
+    %% --- Disk & Mount Services ---
+    FETCH_ONLINE --> kargs_service["ignition-kargs.service"]
+    kargs_service -->|kargs changed| reboot_kargs["Reboot & restart from top"]
+
+    kargs_service -->|no changes| disks_service["ignition-disks.service"]
+    disks_service --> diskful_target["ignition-diskful.target reached"]
+    diskful_target --> mount_service["ignition-mount.service"]
+
+    %% --- Files ---
+    mount_service --> files_service["ignition-files.service"]
+    initrd_root_fs_target["initrd-root-fs.target"] --> afterburn_hostname_service["afterburn-hostname.service"]
+    afterburn_hostname_service -.-> files_service
+    files_service --> quench_service["ignition-quench.service"]
+    quench_service --> initrd_setup_root["initrd-setup-root-after-ignition.service"]
+    quench_service --> complete_target["ignition-complete.target"]
+
+    %% ===== STYLING =====
+    classDef service fill:#42a5f5,stroke:#1565c0,stroke-width:2px,color:#000
+    classDef target fill:#ffa726,stroke:#e65100,stroke-width:2px,color:#000
+
+    class setup_pre,setup,fetch_offline,fetch_service,kargs_service,disks_service,mount_service,files_service,quench_service,initrd_setup_root,network_config,networkd_service,afterburn_hostname_service service
+    class diskful_target,complete_target,network_target,initrd_root_fs_target target
+
+```