docs(assets): how to reach Asset Versions and per-version claims in the UI

[devGregA]

Documentation for the UI half of Asset Versions, which is the Pro-side batch partner of this change.

Phase 3 of the asset model shipped AssetVersion, BOM snapshots and per-version found_in / fixed_in claims, and #15629 documented all of it under Working with SBOMs. None of it was reachable from the app: the surfaces existed only on the API. The Pro change this ships beside adds them to the UI, so the docs need to say where they are.

Deliberately thin, and cross-linking rather than restating. The model, the supersede-rather-than-merge snapshot rule, the no-ordering rule and how imports record claims all already live in Working with SBOMs, and that stays the one place they are explained. The new section in the Assets page covers only what a user cannot find out from the API docs:

• Versions and Affected Versions are optional page-grid tables, so it says how to add them (Page Layout, Edit Layout, Add Widget, Data Table, pick the record set). Neither appears on a default layout.
• A version can now be declared by hand, ahead of any SBOM upload, which is the case the API-first story did not cover.
• The Import SBOM form takes an optional Version, and what happens when it is left blank.
• Claims can now be recorded and withdrawn by hand, and each row shows whether an import or a person asserted it.
• Removing a version removes the snapshots and claims bound to it (both FKs cascade); the Findings themselves are untouched.

The feature flag requirement is restated once, because a reader landing on the Assets page will not have read the SBOM page first.

One section added to docs/content/asset_modelling/engagements_tests/PRO__assets.md, placed after Asset Identity: Aliases and before Asset Nesting, which keeps the asset-model sections in the order they were introduced. No other file changes.