Document Sapphire and Remix

[matevz]

See what is needed for Remix to work seamlessly with Sapphire. Currently, it only works via "universal" metamask connector and a section on this was written years ago for Emerald. OFC you won't be able to debug in Remix much against Sapphire because it won't reveal confidential information.

Don't forget to mention the transactions and queries are unencrypted and unsigned.

Also document the Explorer➝Sourcify➝Remix flow to interact with a smart contract (in the Sapphire➝Guide chapter?). Describe how a (non-tech) user could communicate with the smart contract, e.g. if the maintainer decides to shut down the dApp frontend and the user is stuck with the contract and wants to withdraw his assets back.