docs: add Base Verify Onchain guide (#1728)

Add an Apps guide for Base Verify Onchain, which extends Base Verify's
Sybil resistance and policy gating to enforcement inside a smart
contract. Base Verify signs a short-lived EIP-712 verification the
contract checks via the SignerRegistry and dedupes on identityHash.
Covers the summary/integration steps, core concepts, the claim flow, a
consumer contract example, the POST /v1/onchain_verifications API,
supported providers/conditions, and the security model. Registers the
page in docs.json under Apps > Guides.

Author: patnir