docs(nixos): Migration context + developer NixOS page

[brickbots]

Summary

Net-new documentation on main for the forthcoming Raspberry Pi OS → NixOS migration. Scoped to fill the gaps the unmerged NixOS PR (#379) leaves on main, and built to merge cleanly when #379 lands (it cross-links to, rather than duplicates, the NixOS docs that arrive with that PR).

Three dev-focused areas:

• In-place migration — the one-time, on-device RPi OS → NixOS reflash. Code is already merged (via feat: NixOS migration infrastructure #433) but was undocumented; this is the bulk of the new material.
• Image build — overview + pointer to the release runbook (nixos/RELEASE.md, arrives with NixOS support #379).
• System update — overview of the channel/binary-cache model + pointer to the NixOS context docs (arrive with NixOS support #379).

Changes

• docs/ax/migration/CONTEXT.md — glossary for a new Migration context: in-place migration, post-update actions, migration gate, migration enable, pre-flight checks, migration tarball, migration initramfs, preserved data, migration progress display — with flagged ambiguities and an example dialogue.
• docs/ax/migration.md — architecture deep-dive: trigger paths (gate vs. enable), the all-or-nothing pre-flight, the RAM-staged initramfs reflash sequence, the point of no return, and gotchas.
• docs/source/nixos.rst — developer guide page covering all three flows; wired into the For developers toctree.
• CONTEXT-MAP.md — new "platform lifecycle" grouping (Migration live, NixOS forward-referenced to NixOS support #379) and their relationships.
• docs/source/index.rst — toctree entry.

Terminology

Resolves the overloaded word "migration":

• Migration now means only the OS-level RPi OS → NixOS event.
• The per-version Raspberry Pi OS data fixups (migration_source/v*.sh) are renamed post-update actions.
• Sibling terms image build (the .img.zst) and system update (the NixOS closure swap) are kept distinct.

Notes

• Cross-links to docs/ax/nixos/ and nixos/RELEASE.md are intentional forward references that resolve when NixOS support #379 merges — not broken links.
• Heads-up for NixOS support #379: its nixos/RELEASE.md / brickbots-attic-setup.md cite "ADR 0001/0004" for the Attic cache while the file is nixos/adr/0001 — a stale reference to fix there, untouched here.

Testing

• Sphinx strict build (sphinx-build -b html -W --keep-going) passes with zero warnings; the new page, toctree entry, and :ref: to the dev guide all resolve.

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