docs: document JSON output for celestia-node version command

## Summary

Document the structured JSON output added to the celestia-node `version` command by [celestia-node PR #5203](https://github.com/celestiaorg/celestia-node/pull/5203) once it is available in a stable release.

## Scope

- Update the existing `celestia version` verification guidance in `app/operate/data-availability/install-celestia-node/page.mdx`.
- Mention `celestia version --output json` and the `-o json` shorthand.
- Note that text remains the default output.

## Blocked by

The first stable celestia-node release containing PR #5203. The current stable release, v0.31.4, predates the merge. Do not document a speculative release version.

Wait for the first stable celestia-node release containing PR #5203, then update app/operate/data-availability/install-celestia-node/page.mdx. Extend the existing celestia version verification guidance with --output json and -o json, and note that text remains the default; done when the guidance reflects the released behavior without naming a speculative version.

Written by the indexing model from the issue text.