Doc: clarify SQLite usage as backend option

[WanYixian]

This is the follow-up of https://github.com/risingwavelabs/risingwave-docs/pull/443

Context: https://risingwave-labs.slack.com/archives/C03L8DNMDDW/p1747282673054179

[stdrc]

This page also have SQLite related content. Actually IMO we don't need to remove this from doc, we can move MySQL to above SQLite, and explictly emphasize that SQLite is only for local test environment.