docs: SIWE tutorial and example

[matevz]

Add a separate demo or an example of a dApp that uses on-chain SIWE for logging in and proving authenticity. A simple MessageBox example in SIWE flavor is here, but we should extend it to sth more meaningful.

Document it in https://docs.oasis.io/dapp/sapphire/authentication and mention it in https://docs.oasis.io/dapp/sapphire/browser.

Depends on oasisprotocol/demo-starter#7