docs(nodeagent): add architecture document

[eriknordmark]

Description

Adds pkg/pillar/docs/nodeagent.md — a new architecture document
describing the nodeagent microservice. The document mirrors the
structure of the existing pkg/pillar/docs/nim.md:

• Overview of nodeagent's responsibilities (A/B baseos upgrade,
  reboot/shutdown/poweroff, health-timer-driven reset, reboot-reason
  reconstruction, maintenance-mode aggregation).
• Key Input/Output — every pubsub topic that nodeagent
  publishes/consumes plus the on-disk state under /persist/,
  /run/global/, etc.
• Components — explained per source file (nodeagent.go,
  handletimers.go, handlebaseos.go/handlezboot.go,
  handlenodedrain.go, plus the maintenance-mode helpers), since
  unlike NIM the agent is not split across packages.
• Control flow — four walkthroughs covering boot/onboarding,
  the periodic timer tick, controller-driven device operations and
  the full baseos upgrade lifecycle.
• Debugging — pubsub paths, persistent files of interest,
  useful log greps, and how to force each transition during dev.

No production code changes; documentation only.

PR dependencies

None.

How to test and validate this PR

This is a documentation-only change. Validation is by review of the
content against the source under pkg/pillar/cmd/nodeagent/ and the
related types (types.NodeAgentStatus, types.ZbootConfig,
types.ZbootStatus, kubeapi.NodeDrainStatus).

No automated test is required.

Changelog notes

No user-facing changes.

PR Backports

• 16.0-stable: No, documentation-only addition.
• 14.5-stable: No, documentation-only addition.
• 13.4-stable: No, documentation-only addition.

Checklist

• I've provided a proper description
• I've added the proper documentation
• I've tested my PR on amd64 device — N/A, docs only
• I've tested my PR on arm64 device — N/A, docs only
• I've written the test verification instructions
• I've set the proper labels to this PR