sdks/index.md: Rework "authentication" section

[bfops]

Also figure out if the SDK interfaces which do this need to change, and change them if so. E.g. should DbConnectionBuilder::with_credentials be replaced with DbConnectionBuilder::with_token? Do we still want to provide a mechanism for persisting tokens to disk or browser storage in the SDK, or should we remove that?

[gefjon]

Done in #188