docs: per-stack Fern products with independent versions and a compatibility matrix

[sbaum1994]

Problem

The three Helm stacks under deploy/stacks/ (self-managed, nvcf-compute-plane, observability) were level-set to 1.0.0 and now release independently from release-deploy/stacks/<stack>/vX.Y branches (#1959, #1963, #1965). The documentation model has not caught up:

• The Fern site has a single global version dropdown. Each published docs version is a QA-qualified tuple of all three stack versions (cp-X-compute-Y-obs-Z), so releasing docs for one stack requires qualifying all three together.
• Every docs version is a full deep copy of docs/user/ (about 90 pages). Cutting observability docs copies the self-managed and compute-plane pages too, even when they did not change.
• There is no compatibility matrix. The only cross-stack version statement is a three-row "Stack release set" table in manifest.md.
• tools/docs-version-sync still models a single qualified release set (--qualification-version cp-X-compute-Y-obs-Z) and its README describes the retired semver-on-main release flow.
• check-doc-version-sync is not wired into any GitHub workflow.

Goal

One docs site with a version dropdown per stack, an always-current compatibility matrix, per-stack frozen copies, and no joint qualification gate.

Design

Use Fern products:. The hierarchy is Products -> Versions -> Tabs -> Navigation; each product carries its own independent versions: list, and versioned and unversioned products may coexist. Verified with fern check on Fern CLI 5.38.0 (the pinned version).

Products:

Product	Slug	Versioned	Source tree
Overview	overview	no	docs/overview/
Self-Managed Stack	self-managed	yes	docs/self-managed/ (dev), docs/self-managed-<train>/ (frozen)
Compute Plane Stack	compute-plane	yes	docs/compute-plane/ (dev), docs/compute-plane-<train>/ (frozen)
Observability Stack	observability	yes	docs/observability/ (dev), docs/observability-<train>/ (frozen)

URL shapes: /nvcf/overview/..., /nvcf/<stack>/... (default version), /nvcf/<stack>/<train>/....

Overview holds the compatibility matrix, quickstart, infrastructure sizing, manifest, image mirroring, release-notes index, local development, development docs, and the legacy NGC-managed section. Legacy full-tree versions (0.5, 0.6.0, 0.6.1, cp-0.20.6-compute-0.4.4-obs-0.2.2) attach to the Self-Managed product with a "legacy" label; their trees are not modified.

A page belongs to exactly one product. Cross-product links use absolute site paths because Fern resolves relative links against the rendering product.

Work items (single PR)

1. Docs tree split. Move docs/user/ into docs/overview/, docs/self-managed/, docs/compute-plane/, docs/observability/. Rewrite cross-product links to absolute paths. Add docs/overview/compatibility-matrix.md with a generated block.
2. Fern. Replace versions: in fern/docs.yml with products:. Move version navigation files to fern/products/<product>/<version>.yml. Add redirects for every moved slug and for legacy version prefixes. fern check must pass with broken-links: error.
3. docs-version-sync. Allow generated outputs under the four product trees. Add a compatibility: catalog block and a compatibility-matrix renderer that emits stack versions, source tags, docs versions, and declared compatible ranges. Replace --qualification-version with per-stack freeze inputs. Update tools/scripts/cut-docs-version.sh to --stack <name> --train <X.Y>, copying only that stack's tree. Fix the README release-flow section. Add tests.
4. CI. Run tools/ci/check-doc-version-sync in fern-docs-ci.yml as a blocking check.
5. Stack inventory continuity. Add a test-github-release.py case proving a tag pushed from a release branch resolves the same publisher and resolved_inventory_asset as before. No behavior change expected; attachment is keyed by tag.
6. Policy docs. RELEASE.md, docs/dev/github-release-process.md, deploy/stacks/AGENTS.md, docs/AGENTS.md: the 1.0.0 level-set, independent trains, the N and N-1 support window as the upgrade-stop rule, and the per-stack docs cut.

Out of scope

• Cutting the first 1.1 frozen docs per stack (follows once each train has a release branch).
• Build-time assembly of frozen docs from release branches instead of committed copies.
• Renaming or re-tagging existing stack releases.

Acceptance

• fern check passes.
• go test ./... in tools/docs-version-sync passes; tools/ci/check-doc-version-sync passes on the branch.
• python3 tools/ci/test-github-release.py passes.
• Preview build shows four products, with independent version dropdowns on the three stack products.