docs: add an operator guide for boot interfaces and DPU modes

Operators have historically configured little around DPU modes and network
segments -- the defaults handled the common managed-DPU host. Now that NICo
supports DPUs in NIC mode, integrated and zero-DPU NICs, booting an
integrated NIC while keeping its DPUs managed, and live DPU-mode flips,
operators need to understand both the knobs and what the defaults do when
nothing is set.

This adds docs/provisioning/boot-interfaces-and-dpu-modes.md, the deep
companion to ingesting-hosts.md. It covers:

- The two independent axes -- DPU management (dpu_mode) vs. the boot/primary
  interface -- and the network segment types (Admin overlay, HostInband,
  Underlay, Tenant), with "the host-OS segment follows the boot mode."
- Configuring via Expected Machines: the dpu_mode and host_nics (primary,
  network_segment_type) knobs, and the default behavior when nothing is set.
- Scenario recipes: zero-DPU, DPU in NIC mode, an integrated NIC with the
  DPUs still managed, and flipping a DPU to NIC mode.
- The admin-cli + Forge gRPC surfaces and the web UI capabilities.
- Behind the scenes: the machine-controller state machine and how a boot
  device is chosen and set, plus the predicted -> managed -> retained
  boot-interface data model and the selection precedence.

Wired into docs/index.yml under Provisioning (Day 0).

Part of #2740 (epic #2660).

Signed-off-by: Chet Nichols III <chetn@nvidia.com>

Author: chet

docs/index.yml

@@ -108,6 +108,8 @@ navigation:
           path: provisioning/ingesting-hosts.md
         - page: Ingesting Hosts (REST API)
           path: provisioning/ingesting-hosts-rest-api.md
+        - page: Boot Interfaces and DPU Modes
+          path: provisioning/boot-interfaces-and-dpu-modes.md
         - page: Machine Validation
           path: provisioning/machine-validation.md
         - page: SKU Validation