docs: add backend configuration to metasrv doc

[lyang24]

What's Changed in this PR

similar to https://github.com/GreptimeTeam/greptimedb/pull/4573 This pr adds examples to the backend field in metasrv config in the website

Describe the change in this PR

Checklist

• [ ] Please confirm that all corresponding versions of the documents have been revised.
• [ ] Please ensure that the content in summary.yml matches the current document structure when you changed the document structure.
• [ ] This change requires follow-up update in localized docs.