Add signSubmit() link

Author: ZigaMr

docs/build/use-cases/trustless-agent.mdx

@@ -219,6 +219,7 @@ Inspect on-chain activity and app details in the Oasis Explorer:
 [machine-logs]: https://github.com/oasisprotocol/cli/blob/master/docs/rofl.md#machine-logs
 [sdk-deploy-logs]: https://github.com/oasisprotocol/oasis-sdk/blob/main/docs/rofl/workflow/deploy.md#check-that-the-app-is-running
 [Oasis Explorer]: https://explorer.oasis.io/testnet/sapphire
+[`signSubmit()`]: https://github.com/oasisprotocol/oasis-sdk/blob/main/docs/rofl/features/appd.md