added openrpc api reference

Author: LandauRaz

components/Starknet/modules/references/nav.adoc

@@ -1,3 +1,4 @@
 * https://www.starknet.io/developers/[Developers Hub ↗^]
 * https://docs.cairo-lang.org?_=60[Cairo Docs Hub ↗^]
 * https://governance.starknet.io/[Governance Hub ↗^]
+* https://github.com/starkware-libs/starknet-specs/blob/master/api/starknet_api_openrpc.json[OpenRPC API ↗^]