fix: normalize Hugo page bundles

[Matts966]

Summary

• normalize source layouts where foo.md and foo/ coexist into the Hugo branch bundle foo/_index.md
• rewrite links from the moved overview page to its child pages
• fail instead of overwriting an existing bundle index
• propagate archive download and extraction errors
• add regression tests for bundle conversion, standalone pages, and index conflicts

Why

Vald v1.8.0 added docs/user-guides/vald-operator.md together with detailed pages under docs/user-guides/vald-operator/. The web sync currently generates both vald-operator.md and vald-operator/_index.md, which resolve to the same Hugo URL. Depending on archive traversal order, the conflict was either logged and ignored or caused make all to fail.

This change normalizes that source layout immediately after archive extraction, before Hugo content generation begins.

Failed workflow: https://github.com/vdaas/web/actions/runs/32932596482

Validation

• go test -race -tags=mage ./magefile
• synced the current Vald main archive and confirmed only vald-operator/_index.md plus its child pages remain
• synced the Vald v1.8.0 archive and confirmed the same normalized layout
• confirmed overview links resolve to design.md, controller-spec.md, resource-and-topology.md, and test-strategy.md within the bundle