[Form library] Cut over /forms-new/ to /forms/ and remove the legacy pipeline

[widal001]

Summary

After all forms migrate to TypeSpec composition (#TBD-remaining-forms), atomically swap the new /forms-new/ pipeline into the canonical /forms/ URLs and delete the legacy hand-written pipeline. Final cleanup PR for the form-library refactor.

Cleanup steps

1. Move website/src/pages/forms-new/ to website/src/pages/forms/ so TypeSpec forms take over the canonical URLs
2. Move website/src/components/forms-new/ to website/src/components/forms/ (or merge into the existing components folder if its legacy contents were already deleted)
3. Delete hand-written form folders under website/src/content/forms/<form>/ (formA, formB, formC, formD, cd511, key-contact, sf424, sf424-individual, sf424-mandatory); keep index.json updated and consolidate with typespec-index.json if helpful
4. Update any internal links in docs / starlight sidebar / nav that pointed at /forms-new/...
5. Update the umbrella issue / catch-all references

Acceptance criteria

• /forms/<slug> URLs render the TypeSpec-built forms for every form previously at /forms-new/<slug>
• /forms-new/[...] redirects to /forms/[...]
• /forms/library/ - redirect to /forms/
• pnpm build succeeds with no broken internal links (Astro link validator runs as part of the build)
• pnpm test and pnpm checks pass