docs: document v3 kits (#26065)

Add v3 kit documentation for Docker Sandboxes: concepts, usage,
authoring, and kit sets. Link to the upstream specification for
reference details and retain v2 guidance.

Validation: Markdown/style checks, Hugo build, internal links, and
example build checks passed.

Closes docker/sbx-releases#569

@netlify /ai/sandboxes/customize/

data/whats-new.json

@@ -51,7 +51,7 @@
       "product": "Docker Sandboxes",
       "title": "Sign and enforce trusted sandbox kits",
       "description": "Sign kits with cosign-compatible Sigstore signatures, verify keyless or key-based signatures, and reject kits outside a trusted-signer policy.",
-      "url": "/ai/sandboxes/customize/kits/#sign-and-verify-kits",
+      "url": "/ai/sandboxes/customize/kits-v2/#sign-and-verify-kits",
       "published": "2026-08-20",
       "source_prs": [25791, 25896],
       "featured": false