docs: move the model store and model deployment pages into domain dirs

[thxCode]

What type of PR is this?

/kind documentation

What this PR does / why we need it:

Fourteen pages of the model store and model deployment domains sat flat in docs/reference/ — a
directory the conventions define as lookup tables with provenance — plus a runbook in
docs/operation/. They now sit in their own domain directories, the way docs/kv-cache/ already
does:

New home	Pages
docs/model-store/	artifact.md, image-source.md, prefetch.md, node-store.md, peer-sync.md, views.md, operations.md (from operation/model-store.md)
docs/model-deployment/	deployment.md, prefill-decode.md, routing.md, metrics.md, status.md, shutdown.md, engine-versions.md

• Short file names and H1s without the Reference suffix, mirroring kv-cache/ (backend.md ↔
  "KV Cache Backend"); docs/reference/ keeps the four true lookup pages.
• docs/README.md's page table regrouped into domain blocks; reading paths updated.
• Every pin on a page path rides along: the owned-key test's _ModelDeploymentDocsPath, the api
  comment that generation copies into generated.proto / OpenAPI / applyconfiguration
  (regenerated), the docs skill's routing table, page map and conventions, the overview and e2e
  skills, and every cross-link and link label. specs/ records stay untouched.
• The docs skill's conventions gain the domain-directory row the kv-cache/ split never got.

Which issue(s) this PR links to:

NONE

Special notes for your reviewer:

Verified:

• make lint docs, make lint, make lint agents-shell, and check-docs.sh (49 pages; links,
  anchors, headers, index labels).
• TestModelDeploymentOwnedKeysDocs and RACE=false make test — full suite green.
• make generate reproduces the committed artifacts; the only code deltas are the test constant
  and one doc-path mention in an api comment.
• Stale-path sweep: no occurrence of the old paths or old titles outside specs/ (frozen records,
  per the docs skill's own policy), including line-joined label phrases.

Not run: e2e — the change is docs plus those two inert code deltas. make test with the race
detector fails in pkg/utils/certs/cache (Test_k8sCache_TamperedSecretIsLoggedAsKeyValuePairs,
alitto/pond invokeTask); it fails identically on clean main (5cfac77), so it predates this
branch.

Does this PR introduce a user-facing change?

NONE

[gpustack-code-review]

✅ OpenCodeReview: Review complete: 0 finding(s) across 1 selected item(s).

Reviewed 5320ac8..7dcadea only; earlier commits in this PR were reviewed in a previous run.

[thxCode]

Review round 1 — triage and outcome (answering the OpenCodeReview finding routed to this summary; no inline threads exist):

• docs/model-store/prefetch.md L107-108 — fixed. The lowercase prose link label "node model store reference" survived the title rename (the sweep covered the Title-Case form; this instance was both lowercase and line-wrapped). The label now reads Node Model Store, matching the page's H1, per the suggestion. Landed as a fixup commit.
• Same root cause swept: a repo-wide, line-joined, case-insensitive scan for every old-title phrase (model deployment … reference, model artifact … reference, node model store reference, engine versions reference, …) across docs/, .agents/skills/, README.md and AGENTS.md now returns zero hits.
• Examined and kept: two generic descriptive labels pointing at renamed pages — [status reference] in docs/architecture/topology-aware-scheduling.md and [reference] in docs/model-store/operations.md. Both read exactly like that before this PR (they are descriptions, not the old page titles), so they are pre-existing style rather than rename remnants; leaving them keeps this fix minimal.

Verified: check-docs.sh (49 pages) and make lint docs green; the fixup touches one markdown label only, so no generation input changed and verify-generated re-runs on push as before.