chore: snapshot docs for v3.0.21

[bokelley]

Replacement for #5799, regenerated on current main after the v3.0.20 and v3.1.1–v3.1.3 snapshots merged.

• adds the immutable dist/docs/3.0.21 snapshot from the original release tag
• updates the existing 3.0 navigation entry to 3.0.21 without removing newer snapshots
• preserves every already-merged release snapshot

Validation: release-docs navigation tests (5 passed), full docs navigation validation (21 passed), and git diff --check.

[aao-secretariat]

Clean release-train snapshot. Additive-only: 377 new files under dist/docs/3.0.21/ plus a navigation retarget in docs.json — no existing snapshot is mutated, so the immutable-artifact invariant holds.

Things I checked

• Nothing existing is edited. Every file in the diff is added except docs.json (modified). scripts/check-immutable-release-artifacts.cjs only fires when a versioned dist/** path existed at base AND carries a release tag; 3.0.21 is new, so existedAtBase is false and adding it is permitted. No 3.0.20 files touched.
• Nav retarget is a pure version bump. Base docs.json:75 points the "3.0" version at dist/docs/3.0.20/*; head points it at dist/docs/3.0.21/*. That is the entire 306-deletion delta — path prefixes, not structure.
• Nav integrity — no dangling refs. The one way a snapshot PR breaks is a nav entry pointing at a file that wasn't added. tests/docs-nav-validation.test.cjs:88 resolves every page ref to a .mdx/.md on disk, and L131-142 enforces a single version prefix per version (catches a stray 3.0.20 left in the block). Spot-checked 9 targets spanning every top-level section (building/schemas-and-sdks, building/verification/aao-verified, registry/index, creative/task-reference/build_creative, governance/campaign/tasks/check_governance, sponsored-intelligence/tasks/si_get_offering, trusted-match/surfaces/ai-assistants, media-buy/task-reference/get_products, reference/versions) — all present as added files.
• No changeset required. Diff touches dist/docs/** and docs.json only — none of static/schemas/source/**, mintlify-docs/reference/**, docs/reference/**, or build scripts. No wire surface, no versioning-tracked change.
• Decision class: Editorial (release-train metadata + immutable snapshot copied from the release tag). Replacement for #5799, regenerated on current main after the v3.0.20 / v3.1.1–v3.1.3 snapshots merged; newer snapshots preserved.

Minor nits (non-blocking)

1. docs.json renders as +0/-0 on the API. The patch is omitted for size, so the retarget isn't reviewable from the PR files view — I diffed base vs head refs directly to confirm it's a clean 3.0.20→3.0.21 prefix swap. Nothing to fix; noting it so the next reviewer doesn't read the empty patch as "no change."

Nav validation is the load-bearing gate here and it covers the only real failure mode. Snapshot is additive, immutability preserved.

LGTM.